How do you keep the adults out of it?
If you can't keep the adults out of it, then don't the "Smaller sized or less complex addresses" mean "less secure and easier to steal from the kids"? Wouldn't the difficulty quickly increase to match the cost of the hardware that the adults could afford to mine with?
My idea is to make it so these coins can never really go up in value to much. Something like a trillion total coins. This way they will never seem/be profitable enough for adults to even consider. Being easy to mine will also reduce the possible value significantly. Besides that adults will have 'bigger fish to fry' with all the other more profitable coins out there.
I am talking little kids who have piggy banks now. The 'whales' in kidcoin will have $50 worth and the 'super rich' kids may have 100!
And kids could use their coins to pay for video games and toys not "Silk Road products"..
And when they 'grow up' and need more room to store their wealth they can trade kidcoins for bitcoin(something like 50,000 kidcoins for 1 BTC) or one of the other coins that may end up beating out bitcoin in time...
Again, just tossing ideas out for people to contemplate.