The use of “urchin” to refer to children is separate from its original meaning.

Maybe it became that as a word for something underfoot?

Thats a street urchin. Strangely, this blog post was one of the first links that came up. It ponders how the name street urchin came to be.

It says

Looking in the OED, I see two possibly relevant definitions. 1c. A goblin or elf. (From the supposition that they occasionally assumed the form of a hedgehog.)… There is also 4a. A pert, mischievous, or roguish youngster; a brat.

Edit: formatting is crazy

In French, oursin (urchin) seems to be the diminutive of ours, which means bear. So oursin means something like “little bear”.