Make everything pretty direct and stick to basic doscription and don't get too detailed. If you just say the big tree with thick branches that's fine instead of discribing what type and all the leaves etc etc kids really don't care unless it's directly related to the action.
Have a good irritating antagonist so that when the protagonist over comes the kid is like "heck yes and that's why ___ always wins!!!"
Treasures, pirates, something maybe a little scary, magical kingdoms. It depends a little. Adventures or fairy tales, something with a main character that they can relate to and think "gosh, I want to be him/her". Don't make it too complicated, even if they're obviously not dumb or anything, like, something too complicated probably would lose their interest after a while.
Have a good irritating antagonist so that when the protagonist over comes the kid is like "heck yes and that's why ___ always wins!!!"