My daughter brought two 9 week old kittens into our home, a week ago - born in a barn, barely handled, having been fed (by their "breeder") adult cat food for 6 weeks prior. They were in a pen for two days, because we have a dog and a 13 year old neutered male. Dog has decided that she's their mother, older cat will come and stare at them for a few minutes at a time, then saunter off and make sure he's high enough up that they can't pester him.
Both kittens were very nervy on day one. Day three they were out of the pen and as I type on Day 8, they're rampaging up and down the stairs with the dog watching over them. One of them runs to the dog if he feels uncertain about anything, and naps with her on her bed. The only real problem we've had with them, is the fact that the female kitten was very lethargic on day one, and wouldn't eat (we've gently weaned them from the adult cat food onto kitten food) due to an infected bite wound to her neck, which was already partially healed over, and which the "breeder" claimed to know nothing about, despite having wormed them a few days earlier and checked them over before permitting them to leave. She's fine now, though.
It's not always necessary to make slow introductions - certainly, I've had cats for over 20 years, and have never hidden kittens away in separate rooms to the older cat(s). In my opinion - and I'm sure most of you won't agree with me - sometimes that can only lead to serious problems between older and younger cat. You don't get a puppy and hide it away from an already resident dog, do you...?!
It does sound as if these kittens are completely feral, though, which you should have been warned about. Also, whilst a lot of cat charities do prefer their kittens to be neutered and spayed before being adopted, due to Covid, it's likely that they aren't able to. My daughter spoke to our vet about this when we took them for their health-check/first jabs on Monday, and was told that it's more usual to perform the op when a kitten is 2kg in weight, as it's less risky (anaesthetics wise) for them, which usually means around the 16 weeks category - although they will do it earlier if they're 1.8 or 1.9 kgs and it's a sibling relationship (ie, brother and sister - which ours are).
But yes; completely agree with PP, these are living beings, babies - not toys. If you have a cat already, your children and husband ought to be aware of how to handle/not terrify them. Let them settle in, give them a few days, see how it goes - then make a decision. And remind your children that if they were frightened, they wouldn't like being grabbed at either, would they?