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.