Unfortunately you cannot tell the sex at hatch unless the parents are set up genetically for that. If you are interested you can read the first post in this thread
https://www.backyardchickens.com/forum/viewtopic.php?id=261208 , but the parents have to be set up right for you to feather sex or get red or black sex links. None of those methods work with just any old chicks. You can try vent sexing but I'm not sure how accurate that would be without training.
If you have breeds that Tadkerson lists in that post, you can tell the sex of the chicks at hatch. So that's the best way if you have the right breeds.
I honestly don't know any other way to sex them at hatch for most of us. There are other ways related to sex linked genes but the parents still have to be set up right genetically.
With some breeds or chicks I can pretty well tell the sex at 4 to 5 weeks just based on appearance. But some breeds are harder than others and some chicks of the "easy" breeds are harder than others. I've had some I could not tell for sure at 8 weeks.
I don't have a clue as how to keep your kids from getting attached. I don't know your routines or your kids. I've worked with a lot of kids at the playground, in Scouts, and at school. Most of them can handle a lot more than you'd expect, especially if they know what's coming. My kids raised a lot of pets, gerbils, fish, salamanders, rats, and others. Sometimes those pets died. The kids got over it.
I commend you for thinking of it. You might be surprized at how many people don't realize they can hatch roosters as well as pullets and don't have any kind of plan at all.
Hatching eggs and raising the chicks can be a great experience for kids. My suggestion. Be honest with your kids and let them learn something about life.