A 50 cc mini Moto would be fine for kids, as long as they stay
off of main streets. They are actually quite underpowered, and have
no gears so kids should be fine on them.
Often it is possible to buy a mini moto bike at a regular
motorcycle salesman. You go to the salesman and ask him for a mini
moto bike. Either he has one in his assortment or he asks his
delivery man for a catalogue which he can show to you.