I second Woom bikes (if you are in the US). They are well made, lightweight, proportioned correctly for kids and have phenomenally good resale value. My kids had the Woom 5's and it was a great bike. I sold them each for ~$50 less than what I bought them new for, and they both sold within a few days.
Before I discovered Woom I bought a succession of Specialized HotRock bikes for them- 12" wheels, 16", 20" . These are decent bikes that are easy to find used on CL. But I did have to do a bunch of work to lighten them and make them more useable. These Hotrock bikes had four shortcomings: too heavy, geared too high, reach too far and crank lengths too long. After I swap out the tires, stem, handlebars, crankset, chain ring and cut off the kick stand mounts, I manage to make them light and kid friendly. Luckily you can still find a lot of BMX parts to customize and lighten these Hotrocks to your liking.
But if you just want something that works out of the box, Woom is it.