Comment by CRConrad

Those were already somewhat common around here, back when I taught my kid to ride some... 17-18-19 years ago. But it felt rather superfluous to buy a separate gadget that he'd use for at most a few months, when the same effect could be achieved by just taking the pedals off a "real" bike.

(Unfortunately, we'd already started him on the old-fashioned route with pedals and training wheels. Fortunately, he wasn't all that heartbroken when we "discovered that a thief must have stolen" the pedals and training wheels. And he was quite ecstatic when later "the thief must have returned the pedals".)

So I really recommend the ordinary-bike-with-removed-pedals method to start with. The trick is just to get the saddle low enough. Old-fashioned[1] saddle mounts with a multi-piece clamp on the saddle post usually have that below the longitudinal rails on the saddle. You can flip the bits around so the rails go below the bolt of the clamp, and thereby lower the saddle by a couple centimeters / about an inch. This is a rather fiddly job within the narrow confines under the saddle / above the rails / inside the side flaps of the saddle, but it's possible. Three guesses as to how I know this.

___

[1]: And most stuff on not-exorbitantly-expensive kids' bikes is quite old-fashioned; kids' bikes are much cheaper than "serious" adult ones, so manufacturers have to scrimp wherever they can.