We have friends who raised bantams right alongside standards from chickhood, and never had any issues at all. We're doing the same with our Cochins and Silkies: raising them alongside the Orpingtons and Wyandottes. If they are together from day one, they consider each other to be siblings and part of the same flock. Check the new issue of Backyard Poultry. There's an article about a family raising Silkies, an Americana, and a Buff Orpington together. They brought the chicks home together, and from the photos it looks like they're one happy family. If you're concerned about the one bantam in a box of standard chicks, though, order more than one. That way, the bantam has some company and you have a back up.
Oh, about bantam personalities (sorry, forgot this when I posted). It's going to vary from breed to breed. Seremas are active, OEs can be flighty. Cochins are docile, so it's a good breed for your daughter.