For some reason, I wasnt able to say much beside :
-Its decentralised, its a «store of value», it can be seen as «property» (as opposed to land or house or anything that a govt can seized from you), and that all transactions or public to everybody via Blockchain.
You only make it more complicated, how can a kid understand bitcoin for all of these you have in mind? There's no way a kid can understand such statement or I don't think you should tell a child about bitcoin or blockchain.
A simply "alternative money that can be transferred online" is much easier explaination for their age, technicals can be learned when they're at the right age to learn more.