Well, the typical bright colours with thick lines and happy faces everywhere is always a good bet when dealing with that age group. Basically, cartoony > realistic. Being a dude, i can say that the male children a probably a bit more indifferent about the style than one should think (i mean i played Doom at that age), but do consider your choice of colours. Society says that more pink = more girly (minus the hell barons in Doom, they're anything but girly), while green and blue are traditionally more boyish colours.
An excellent example of something appealing to kids of that age would be Super Mario. I have a grandcousin (i think it is, cousins son) who's 6 (i think) years old, and he played Mario on his DS, which he enjoyed.
Also, huge shiny eyes (you know the type) with otherwise tiny facial features are typical in more girl-oriented things, while boys like big manly heroes with huge chins.