I honestly don't see the problem. Five year olds will not vote, anyways. There will of course be laws (if they dont already exist) against parents forcing their children to vote they way they want to. There will be so few five year olds voting that it would not effect the results. In fact, nothing bad could come of it, you're just worried about nothing. Think outside the box. Of course if this were to happen, and there were problems with it (that could not be fixed by strict laws) then I will say the voting age should still be very low.
Any laws preventing such abuse wouldn't be effective