I'd check dell outlet for a refurb. Minecraft doesn't take much and a lot of tablets/smartphones can run it. I don't know what roblox is though. It may be better to get a better tablet like an nvidia shield k1 and get the controller. Then you have the option of hooking it up to a tv and it has game streaming
It has an AMD A8, 8GB RAM, and 1 TB HDD. I've had laptops with AMD A8 and A10 processors and they've exceeded my expectations for gaming. They basically have integrated graphics that pull from the system memory when playing games and do really well. I played stuff like Sleeping Dogs on one, so I think Minecraft should be no problem.