"Theme Park Tycoon" is a Rollercoaster Tycoon clone that's really nice. It's pretty much single-player, but you can check out other players' theme parks which is very cool.
"Islands" is mostly a Minecraft ripoff but you're building air castles (you can fall off an "island" and die). This lets the kids do all kinds of invented in-game games, such as building an obstacle course for one another, playing hide and seek etc.
"Fishing Simulator" starts out as a pretty dull pirate-themed fishing game but turns out to have a lot of depth, adventure game style. It also has amazing content for Roblox standards.
"Build A Boat For Treasure" is a lot less fancy than the above, but it has a very cool concept. You design & build your own boat, then go sit in it, and take it through a river full of obstacles. The way you build it determines how it deals with the obstacles because when you sail away you can hardly navigate it anymore. So you need to design for robustness etc, which turns out to be pretty hard.
A lot of the games are shit, but also a lot of the games look shit but have super original ideas. Eg there's a game that you're in a building and then some disaster happens (tsunami, volcano eruption, whatever) which slowly destroys the building and whoever survives the longest in the collapsing building wins. I'm really not sure if that's a ripoff of anything, it strikes me as genuinely original. It's not very deep, but it's good fun.