5 Year old rolls on a Spawn Gremlin 14" and loves it (it's pink, that's usually the most important feature for her). Probably time to get her on a 16" but I keep waffling on that or just keeping her on the 14" until she's big enough for a 20".
3 year old is on the Kinder Bike run bike, which is, most importantly to the boy, blue, and most importantly to me, has a hand brake. He's suuuuper impatient to get on a pedal bike and is still too small for the Gremlin I have lined up for him so I snagged him a 12" Cleary Gecko that is waiting for us in Edmonton.
One child trips to school or rips on mellow trails the Norco Yukon with MacRide gets the call, and soon we'll have the Wildcat setup for MacRide rips too so we can hit some "serious mountain biking trails" as the boy likes to call 'em.
Two kid trips and the Big Rig gets the call: RadWagon 4 with brake, handlebar, stem, and seat upgrades, a Caboose, a rear bench seat, a Yepp for the boy boy, and custom fir box for the front bag and rack. An ancient Roach messenger bag is undergoing some alterations so it can go on the back of the Caboose, giving the lock and kids helmets have places to live out of the way of the cargo bag out front. Bit of a POS but it's working well for us now.