I recently bought my son, who will be four next month an Oset 12.5V Trials Bike, what a great piece of kit.
They are battery powered, so no hot engines or petrol/oil to deal with, the battery lasts about an hour & a half at half speed, its fitted with a speed controller so you can adjust to suit, max speed 14mph! Two larger options are available 16, 24v & 16, 36v.
We hadn’t put Archie on a bike without stabilizers before the Oset, but he jumped straight on & took to it immediately. Next stop some school boy’s trials to show him what he could be doing when he’s a little older!
Just a heads up if anyone is looking for some clean fun for their kids or grandkids.
PS: Sorry for the shameless proud Dad post!

PPS: I have no association with Oset, or trading partner.