Before that if it's too much, they could make things like shirts to wear in game and sell those to other players, easiest is to use a template and something akin to photoshop.
Roblox is closer to a game marketplace where the games are made by fans than a game unto itself, so the creativity isn't game inherent unless it's subgame was built to be like that (most aren't).
To share my own example - beyond SimCity and not-really-understanding-how-to-make-Age-of-Empires-maps when I was 10 years old - I don’t think I can say I was really using computers constructively until I was 12/13 when I was at the point where I actually understood what I was doing with my cracked copies of Macromedia and Adobe software - so if I’m using myself as a benchmark I wouldn’t stress too much about grade-school kids being newbs…
…just please don’t repress them. And introduce them to the concept of creating their own things outside of closed platforms.
Then around 12 I became way more social online, started making custom skins for InvisionFree message boards and spending more and more time in the CSS Zen Garden... and the rest is history!
Another thing you could try is get an account and play along with them. A lot of games allow for private servers / areas where you can play.
Get them to add real friends from their school and hookup on a web conferencing app. That really helps with team building and communication.
6-7 years old could use it to create obby game with adult guidance.
As a whole, they are no more creative then any other games. And they are rarely if ever as detailed as something like Minecraft.