For a well rounded group in all four roles, and 3 different power sources (Arcane, Martial, 2 Primal):
Striker: Rogue, DEX/CHA build. Skills: Acrobatics, Bluff, Intimidate, Stealth, Streetwise, Thievery.
Leader: Bard, CHA/INT build. Skills: Arcana, Diplomacy, History, Religion, Heal.
Controller: Druid, WIS/CON build. Skills: Endurance, Heal, Insight, Nature.
Defender: Warden, STR/WIS build. Skills: Athletics, Dungeoneering, Nature, Perception.
Result: All 17 skills covered, with some overlap on Heal (with the Druid being the true person for the Heal check, the Bard picking it up for emergencies despite not having WIS primary/secondary), and Nature (shared between Druid/Warden.) Hits all four roles. And likely has a bit of fun in the process. ;)
Going by the roles listed in this answer I would say that you have a fairly balanced party that covers most roles adequately.
A person who can heal
This is one area where you might be lacking a little. The Paladin gets Lay on Hands starting at level 1, and they get spells that include Cure Wounds at level 2. The Ranger also gets spells like Cure Wounds at level 2, however they have a limited number of spells known so they probably can't devote too many to healing ones. I would recommend having someone with the Healer feat if at all possible, or proficiency with a Herbalism Kit to make your own potions.
A person who can do area of effect damage
This is covered by the sorcerer. They can start at level 1 with Burning Hand or Thunderwave, and have more options as they level up.
A person who can tank up damage well
The paladin has this covered, with d10 hit dice, heavy armor and a shield they should be difficult to hit and if they are hit they take a lot of damage before they are removed from the fight. The ranger is also decent at this, also having d10 hit dice and a decent AC.
A person who can deal lots of damage
This can be done by everyone fairly well. The paladin has Holy Smite to boost his damage, sorcerer has spells that do a good amount of damage and metamagic to improve them as well, and the ranger gets the spell Hunter's Mark.
A person who can bypass obstacles easily
This one is a little vague since an 'obstacle' can be just about anything. However you have most of the most common ones. For a party face the sorcerer should be able to do well (and possibly the paladin depending on where they put their stats and proficiencies, though being a kenku their ability to help here may be limited). The ranger should be able to be a good scout with Stealth and Perception.
There are a few places they might struggle however. One being traps and locks since nobody has thieves' tools proficiency their main answer is going to be to brute force it. And the other is knowledge based challenges since they might not be proficient in many of the requisite skills (acolyte has Religion, however the rest might not be covered), and unlike things like being a party face or scout the party probably won't have the intelligence to do well with these checks if they don't have proficiency in them.
A 4th member
My recommendation would be either Bard or Wizard, with a background that gets proficiency with thieves' tools (Criminal or Urchin from the PHB or Urban Bounty Hunter from SCAG), or a herbalism kit (Hermit from PHB). Both of these classes help with some of the skills you are missing and also add some battlefield control that while it isn't a necessary role is helpful. Half-elf or Human would help with getting skills, however this isn't super relevant.
For the Bard they should pick one of the ways to get thieves' tools, since they get spells can heal so they don't have to worry about that. I would say something like Sage to help pick up the knowledge skills. And with Jack of All trades at level 2 they will be somewhat competent at most things.
For the Wizard I would probably choose the Hermit background, because they have spells that can fake the usage of thieves' tools (things like Knock aren't perfect, but I think the party could use more healing). For them while picking up proficiency in some knowledge skills is useful it isn't as necessary since they will have the intelligence to pull them off a lot of the time any way.
Best Answer
This is an opportunity, not a problem
I can understand why you are concerned. I thought much the same in my early gaming days, but with experience I came to realise that it's much less of an issue than you would think.
Roles matter more than stats
For combat balance it is more important that you have balanced combat roles than diverse ability scores. A Dexterity-based fighter is just as effective in melee as a Strength-based one. So long as you have a reasonable balance of melee and ranged, blasters and healers, you should be fine.
Out of combat
Out of combat don't think of low stats as blocks that ruin the campaign, but instead challenges to overcome. In one of my current campaigns, we rolled up a stat block as a group and all used it as a stat array. In that array, there were two 7s. Due to the classes we picked (Druid, Rogue, Cleric, Fighter, Wizard) we wound up with every player dumping Charisma. As you can imagine this regularly poses problems for us in social situations.
To compensate for our low scores, we use magic and clever planning to gain advantage when we need to talk to people. Is it inconvenient? Maybe. Does it ruin our fun? Absolutely not. In fact, it opens us up to some amazing role-playing and hilarious experiences as we try to work around our terrible stats.
Your DM
A good DM will know the limitations of their party and make allowances for it. There's no point setting a Strength DC of 25 if the highest the party can reach is 20. This doesn't mean your low Strength won't matter, but it means that they should be allowing ways for you to get around it.
Additionally they know you built a party of high charisma characters. This often implies a preference for more roleplaying in a campaign, the DM should provide the opportunities for the party to play to their strengths.
Managing your high charisma
Sometimes with lots of roleplay heavy characters it can lead to difficulty in sharing the spotlight. I've played in a party with 3 characters of 16 or higher charisma, we had to learn to share the social skills evenly. We used this to expand our characters, taking on a unique personality and persuasive style based on our backgrounds.
We would then choose who to would talk to any given NPC based on who they were. The shady rogue talked to the low-lifes and criminals, the noble bard talked to the lords and the well-traveled sorcerer did most of our merchant dealings. The important part was that we each felt useful to the party and didn't get in each others way.