Depends who the form is for. Almost all forms I fill out for the kids will have a spot for their name and a spot for the parent's name. But if it just has a spot for the family name and that's it (like on raffle ticket envelope or something) I would put the child's surname as that's who is at the school. I've never come across anything where it was an issue tbh. It may be when DD starts school as she has my surname, but I guess I'll just keep putting DS's surname as he is the eldest.