All D20, but DnD 5 especially, are designed and balanced assuming a 4 or 5 player party. And you can kinda tell, when the rules start asking you to apply multipliers to bigger parties, rather than giving any concrete guidelines. Assuming distributed player competence, a 6 person party isn't simply 20% stronger than a 5 player party. It's much higher. There's an entire additional PC, with their own suite of abilities, magic items, and most importantly, actions. Never underestimate the power of having more standard actions than the other guys (unless you're a pack of CR1/2 minions going up against a bunch of level 5 adventures, then you're screwed either way).

In short, designing challenging encounters for big parties is one of the more substantial challenges a GM may have to face. There seems to be a razor thin design space between "no actual danger" and "guaranteed TPK" when planning for large parties. My own personal strategy is "5 players per party max unless you have a super good reason that a sixth needs to be in this particular game, and never ever ever ever more than that (and preferably not 6 for long)," but that probably won't help you, specifically.

First of all, stop giving them one big thing to focus on. There are a couple iconic encounters that tend to necessitate one big monster against a party of intrepid heroes, like dragon slaying. The problem here with big parties is 1 creature generally can only attack one thing at a time, so even if the beastie is downing 1 PC a round, the rest of the party can pop cool downs and beat the timer. If you design the combat space so the beastie can use it's AOE abilites to good effect, you often find yourself looking at a TPK. Quantity may be a quality all it's own, but it isn't everything. Basically, when the party is that big, the single monster encounter HAS to be able to one round KO any given PC, or it's not a threat. And while it's killing one PC a round, the remaining party members HAVE to have a DPR high enough to whittle the beastie down before it kills them all, or they all just die. SO! Anytime you're tempted to let 7 or 8 dungeon crawlers dogpile one big boss monster, resist the urge. Instead of fighting one wyrm, why not a mated pair of adults, maybe with a wyrmling thrown in to harass the squishies? (Actually, this particular piece of design advice sort of holds true for any size party if there's more than one healer to keep the front liners standing)

Secondly (related to the first), if your party outnumbers the monsters, they'll probably win unless each monster is SIGNIFICANTLY stronger than the PC. I can't give concrete CR equivalencies because it's different at different power bands, but 7 lvl 5 PCs who know which end of a longsword is pointy should mop the floor with 5 CR7-8 monsters. The power of two players worth of extra actions is too substantial to ignore. Design the encounter using the guidelines in the DMG, then add a few support casters or bowmen(or bowwomen, or bowgoblins, or bowwhatevers) at a little under CR to bring the numbers within 1 of the party.

Finally, if you're going to do mob (mob in this context being a large angry group, not a MMO enemy, nor a crime family) encounters, consider looking at the minion style monsters from 4th edition. They had decent defenses and attack bonuses, but 1 HP and very low damage. Custom brew up something mob-able, maybe give them advantage for being adjacent to allies, with a decent attack bonus and a beefy AC, but a damage range of a d3 and very low health. And remember, mob fights aren't typically meant to be challenging in and of themselves, they should be hard enough to drain some resources while letting the players feel like badasses for steamrolling through a pile of enemies. Remember, damage spread to 7 players hurts a party much worse than that damage stacked on 1 or 2 frontliners.

Your PCs will likely not survive the Amber Temple fully intact. Even adding the two Winter Wolves is not likely to help, as they are only CR 3 creatures. That is, one Winter Wolf is a medium challenge for a party of 4-5 level 3 characters.

Compare that to the CR 12 monster in the first encounter, which alone would be a challenge for 4 level 12 player characters! Not only that, but the Arcanaloth is a 16th-level spellcaster, with finger of death and chain lightning both in it's repertoire, in addition to no fewer than 3 castings of fireball available to it. In 5 optimal rounds in this one encounter, you're looking at a lot of damage, certainly enough to kill at least one of your PC's.

Without doing the calculations (don't have my DMG at work...), two Winter Wolves are still only a hard (or possibly deadly) challenge for APL 3. You can't treat them as level 8 characters based on the hit dice, because level 8 player characters have classes which grant abilities at level 1, level 3, level 5, level 6, and so on, with level 5 being the most important as it's where the power level ramps up (fireball and Extra Attack come "online" at level 5). Winter Wolves don't have any of these advantages, and even with Pack Tactics and Cold Breath, it's unlikely to make a difference as the party is shelled by the highly deadly encounters within the Amber Temple.

The Amber Temple is one of two areas in the adventure with a recommended party level of 10, the other being Castle Ravenloft itself. See the Areas by Level table early on in the book (page 6). You can have a raven try to warn them away, similar to the encounter at the entrance to the windmill, but ultimately your players are the ones who get to choose.

That being said, if they are intent on entering the temple, let them. If they survive, so be it. If they start getting wrecked early on, you can hint to them that it might be wise to regroup and come back later when they're stronger, i.e., when they're more appropriately leveled and well-equipped. If they don't take your advice and push forward anyway, let them TPK.