They are actually becoming quite popular again. Better idea than training wheels as the kid learns a sense of balance which is the most important thing.
He can upgrade to a normal bike later.
I don't know why you haven't noticed them before
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Spoiler warning:
I would imagine it would be different, because you're not walking or running, you're pushing with your legs.
And I've never noticed then before because I've never seen one, even when looking at bikes the other day. Either they haven't hit Canada yet, or they aren't big in my town.