Roblox reminds me of a crappy remake of RTB.
They got stretchy bricks, 'impulses', and while going around a few servers I found some of them to be quite similar in concept to some RTB servers.
It's not a true building game, it's more of a complex, laggy, scripting and engineering sort of thing and I fail to see how it is child friendly.
I've also had much more fun playing and building in RTB than Roblox.
TBM was quite fun also, too bad they were a bunch of dirty rats.