I'm thinking about something, maybe kinda kooky, but bear with me. Colorful griptape could be appealing for the kids, even more so than any graphics. I think that, as long as you don't pick a horrible graphic or anything particularly gross or dumb, the little girls will love it. A nice sheet of grip in a bright color would make it stand out for them. Make it look like a fun, colorful little toy. They'll look more at the grip than at the graphic, at least if they use the boards for what they're meant for. And just like the wheels, you can change it when the time is right.