There definitely are child labor laws, especially for those on sets, so it’s entirely possible that maybe they did it as more of a promotional thing when they had LOTS of Ewoks shown on set.
Yeah, it seems like a really odd thing to do for any other reason. The rules vary from country to country, but anything involving children in film and TV is almost never worth the trouble.
If they used kids at all for a “practical” purpose I’d imagine that they only used them for certain shots that required more than the twenty or so Ewoks seen in the photos V.I.N.CENT posted. Though I can’t really remember any scenes that had more than those Ewoks anyway. I checked the ambush scene again just to check, and it looks like they simply showed the same twenty Ewoks over and over again to imply a much larger crowd.