Wild Shape is pretty important to Moon Druid.

I'm currently playing a Moon Druid, and Wild Shape is key to the effectiveness of a Moon Druid in combat. If your players had wanted a spellslinger Druid, Circle of the Moon was not a great choice. Circle of the Land or another Circle that improves spellcasting would be a much better choice.

TL;DR: Yes, as a Circle of the Moon Druid they should definitely be focusing on their Wild Shapes if they want to be most effective in combat. However, you can play the character and have fun without Wild Shaping in combat by slinging spells (particularly healing and control spells such as Cure Wounds, Healing Word, Entangle, Faerie Fire, etc).

Your best course of action depends on why they aren't using the Wild Shape. Are they making the conscious decision not to, whether for story or personal reasons, or are they just not using it because they're inexperienced and not sure how to use it properly? Or, perhaps they aren't using Wild Shape in combat because they see the party composition and they don't want the entire party to be in the front line swinging at the enemy, so they choose to hang back instead.

Now, if they're complaining about their characters' low HP, then I would hazard a guess that it's because they're inexperienced and don't know any better. In that case, I'd pull them aside together (with the DM if possible) between sessions and show them some of the cool shapes they have access to (not sure what level you guys are but Giant Hyena, Dire Wolf, Giant Toad, and Giant Spider take the cake at low levels for me).

Yes, you probably have seen a Brown Bear, Lion, Tiger or Dire Wolf at some point.

If they insist that they haven't seen any of those beasts yet, you'll have to talk them through the process of going through their characters' lives BEFORE they became adventurers and consider what they would have seen during that time. Page 24 of Xanathar's Guide to Everything has a really good bunch of tables on it that they should have a look at. The tables are organized by biome (grassland, forests, mountains, etc) and have a fairly long list of beasts up to CR 1 that your characters would have probably seen during their lives pre-adventuring if they were from that type of area.

Discuss with your DM and those two players and see what you can work out. If you have noticed their complaining and lack of Wild Shapes, the DM probably has as well. If you include your DM in the process it'll go much smoother.

Inspiration/Excitement for your players

--Warning: D&D Math Below!!!---

My MVP wild shape pre-level 4 for prolonged combat has got to be Giant Hyena. I'm going into power-gamer mode for a minute here: doing some basic math, 45HP per wild shape (giant hyena) at 2 transformations per rest is 90HP. If we assume level 3 then you have 4 first level spell slots and 2 second level spell slots, for a total healing amount of 8d8 hit points over the course of 6 turns (one slot per turn), which averages 36 hit points. If you assume that your characters have 12 CON (+1 bonus) and that they take average HP at each level up, they will have 19HP at level 3 on the un-transformed character itself.

Adding this all up, you have 90HP + 36HP + 19HP which is 145HP at level 3, assuming average healing and HP rolls and that you expend all of your resources, and that's without any external assistance such as your cleric or health potions. Show that to your Druid players and hopefully they stop complaining about low HP.

Conclusion

I don't mean to encourage extreme powergaming and it's unrealistic that they would blow all of their spell slots on healing and all their wild shapes in a single combat. I mainly wanted to show how effective the Wild Shapes can make the Moon Druid and hopefully this can inspire and excite your players to get into the next game and try out their Wild Shapes, instead of hanging back and slinging Thornwhips and Ice Knives.

Spellcasting

The sorcerer spell list is significantly more damage-focused than the druid list, which tends towards debuffs and utility spells. That doesn't make it inherently unbalanced as such, but it might be of concern. I might suggest using the druid list, and if there are a couple of specific sorcerer cantrips or spells that seem particularly apt for the character, add them to his list, rather than just switching out the entire spell list. It only takes one or two cantrips to give the character a unique flavor, without going to the extreme of rewriting the whole class list.

Wild Shape

Switching Wild Shape into, let's call it "Monster Shape", is a tough question. Honestly, it really does depend on which subclass he takes, because a Moon Druid's expanded CR choices will make all the difference.

A non-moon subclass may find the monster choices very limiting, because there just aren't that many weak monstrosities that don't have a fly speed. I think you'll find that at 2nd level (CR 1/4) he'll have no valid choices, and only a handful at 4th level (CR 1/2). This may, of course, change with future book releases. Once he hits 8th level there are maybe a few interesting Monster Shape forms, but still very limited, and weak for that level to boot. A normal non-moon druid at that level tends to use wildshape for utility effects rather than to get cool combat forms, and some of that utility will be lost just because a Darkmantle or whatever is a lot more notable than a raven, rat, or even bear.

Using Circle of the Moon adds quite a few forms. It's still very limited, but maybe not so bad; the big issue there is as you level up, you'll be adding monsters with bizarre abilities to the list, some of which are fine for their CR in the context of a single fight but may be gamebreaking if a PC can just use them any old time he wants. Most of the really bad stuff is forever out of reach, but at higher levels you will see stuff like Lamias, Gorgons, and Medusae coming into play. And Mimics are in at 6th level, which all by itself might give you pause.

So what can you do? Well, I feel people often try to over-complicate things by rewriting huge swathes of rules text to fit with a weird idea, rather than taking the safer and easier path of reskinning the existing rules with new descriptions to match the flavor of what they want to do, and possibly making one or two little tweaks when the mechanics don't quite work as written.

My suggestion in this case is to use Wild Shape as it's written, but change the flavor by reskinning each of the Beast forms to describe some quasi-human monstrosity instead. A dire wolf becomes a hulking wolf-man. A giant eagle becomes a harpy or even griffon form. A warhorse form is described as a centaur or minotaur, a brown bear is an owlbear or half-troll, a giant bat is the Man-Bat from DC Comics. Whatever. I think that sort of thing could easily give you the flavor of turning into bizarre monsters without getting into the mechanical difficulties of the actual Monstrosity creature type. (You'll still have the roleplay effects of being a gross monster rather than just a regular horse or what-have-you, so this might work best if he goes for Moon druid.)

The only mechanical changes that might come up are whether some monster forms can talk or use their hands, but even then it's less common than it might look. A giant wolf-man that's secretly a direwolf probably can't speak like a human, just growls and howls, and his massive clawed hands could easily be useless for anything but the most basic grasping that a wolf's muzzle could accomplish just as easily. You could have his forms use the "monstrosity" type rather than "beast" -- the only real game effect there is immunity to a tiny list of spells and effects. ("Animal Friendship" and "Dominate Beast" are the only ones that really come to mind, off the top of my head, and those can be largely duplicated by higher level spells -- they're just easier to get to if you're targeting a beast.)