That's precisely what it is. The game animations are fairly simple, but you can build a world and write fairly sophisticated game elements/rules/etc with Lua, and the kids actually seem to like the cartoonish models.
Once you write a game or create a world, you can click and publish it for anyone to join.
Many of the games are similar-ish to popular games but the barrier to entry is low, share assets, games are easy to jump in and out of together with your friends.
It is basically that. With freemium style monetization, cooldown periods, timed pop ups, consumable items (purchase in the store) and all,
My niece was about 300 dollars into this game before my sister got involved. She was using 'oh get me itunes gift cards' from relatives so mom would not notice. She was using all of the tools of an addicted person to use that game. Her sister on the other hand got bored with it and gave up on it.
I haven't played roblox and know relatively little about it, but my guess would be that it is popular with children because it has a low barrier to entry. Sort of like what Flash did for young animators.
Kids love the total mess! As an adult I don't see the appeal but I can understand why kids like it. Most of the complaints here about Roblox and revenue sharing, etc, completely miss the point: Roblox is not Steam; it's a playground.
Once you write a game or create a world, you can click and publish it for anyone to join.