So sorry about your little girl, but as far as what I know about fontanels, the puppy can still go on to live a full, active life - as rambunctious as any other pup. I know there are other members on here whose pups have an open fontanel and they are just like any other puppy - dogs aren't even aware of it. I say as long as your daughter understands the situation and how sensitive this 'soft spot' is, then let her take care of the puppy. Just make sure she knows the responsibility that she is taking on - that it won't be quite the same as just having a new puppy running around, that extra precautions may need to be taken with her.
Good luck in your decision! Hope your baby girl is well!