Are any flights currently booked? If not, one way to accomplish this is to set up everything as one way tickets.Has anyone ever booked flights this way? My daughter is trying, but has a call into Delta because while booking online it makes it look as if the 4 year old child is flying alone.
If your granddaughter is booked on a separate reservation from her parents, she'll have a different passenger name record or confirmation number. Your daughter can always contact Delta about linking the two reservations.
As for the price change, this may come down to the whole "prices are not guaranteed until ticket is issued...".
Good luck, I hope this works out for you.