There doesn't seem to be any Roblox fabric that's for sale. The only thing I can think of is a large applique. They characters are square, so it shouldn't be that difficult (especially since you are doing it and I'm not). If you'll go to their site, they show their characters and their assorted costumes. If I did it, I would make one large character on the quilt and a pillow cover that matches the background fabric. Look at the youtube videos to get familiar with the characters.
Let us see what you do. I have a young neighbor who would love a roblox quilt, so I'll keep my eye open.
bkay