Children being dogmatic!

Hardly up to you to resolve this surely? Meant to be pleasant for you, a weekend away.
Not ideal to put a young puppy in kennels unless a true emergency.
Although it would be nice to have your DD there, would it be a compromise too far to have a weekend with your boys and DGC and later a lovely day / weekend with DD and her puppy?

I'd seen the crate being in the daughters bedroom, door firmly closed. The dog coming out for exercise would involve a lead and walk outside. It's manageable, it depends whether the will is there. I wouldn't want to leave a puppy in kennels. If the pup is very young, it's possible the breeder would care for it over the weekend, with kennels costs given of course.
The other option is a friend of the owners moving into her house, so the puppy isn't unsettled during key stages of its development.

I think it could be pretty stressful for all concerned. The puppy with all the children fussing, the owner worrying about the puppy and the parents worrying about the kids and the dog and you in the middle. If the puppy is old enough to go in kennels that's the best solution or perhaps have a dog sitter in their home while they're away.

In my view, a crate would not be a good idea as there would still be a chance of children getting too close, if adults were not keeping a very close eye on them.

Also the dog would have to be taken out of the crate for exercise, giving opportunity for mishaps.

Kennels.

I think it might be too many children for a puppy to cope with at once. Not that I'm any expert, but better safe than sorry, I think.

I agree with those who say no to the puppy, small children and small puppies don’t mix well on a short term holiday basis. Yes they can all learn but not over a weekend....which is supposed to be fun for everyone!

I'm astonished that the event has been set up without getting this sorted out first.

Definitely kennels for the dog.

Have your son and daughter talked with each other about their different hopes for the birthday celebration, or just given their mum the problem to resolve.
If the puppy is well trained its likely/possible it is crate trained. If so, the puppy can happily retreat to its crate (door firmly closed) and the children told to leave it alone. Children do need to learn how to behave around dogs, just as dogs have to learn to behave around children.
Adult children also benefit from learning to compromise and try and avoid family dissent around significant birthdays.

I would put the puppy in boarding kennels or find a good dog sitter. However good the puppy is, and however well trained it is still a dog, will all the instincts of a wolf. I have first hand experience of a child who is nervous around dogs. My sister, she grew up into a woman who is still afraid of dogs, even though I have had several lovely gentle dogs, she is still nervous, and it all stems from one incident in her childhood. She wasn't hurt by the dog but frightened by the jumping and barking. If your DGS is nervous or afraid it isn't worth the risk.

That sounds rather a nightmarish weekend - four little ones 4 years old and under - and a young puppy who is presumably not used to small children and one child, at least, is not keen on dogs.

Trying to stop small children enthusiastically 'loving' a kitten or puppy is difficult at the best of times, but when you are trying to enjoy your special birthday it is a distraction too far.

Can your DD find someone to look after the puppy for the weekend if it is too young to go into kennels?
I would definitely not want her to bring the dog.

I would not want a small puppy around children who don't like dogs to start with.
Could you find a nearby kennels so you can visit the dog and walk it under control when you are not in the party setting .

I would not put a small dog who is not used to children into this scenario. From what you say there are three children under four who do not know this dog.

Very small children do not have the understanding to manage their behaviour around dogs and neither do excited small dogs.

Your DS has every right to say this. He is protecting his family. Pay for kennels for the dog - it's only a few days - it will live.

Have a good significant birthday! How lovely that this has been arranged.