I got the sarcasm in the previous answer...
I guess what
@Star-Dust meant was, there's no magical answer...
There are however methods to help come up with an idea...
The main one is BRAINSTORMING!
Gather a bunch of kids within the age range you've like to target your game at.
Let them express what kind of games they play, what they like about them, and mostly what they would change or add to them.
Them with all that info, and a good notion of your real skills, start putting scenarios into paper... and the game base will arise from it.
Another, less scientific, approach, is to create a character and build the game into it... (I made no mistake there!)