Earlier today i sold my Wii U that I've had for over 2 years because i don't play it much. But my sister who is 8 played it every day. She likes straight forward games that aren't complicated. Games like Splatoon and 2D Mario. Since i only have my Xbone now she would probably wan't to play it so i download a free game i found called Roblox and she really likes it so i need easy games that are fun. Also they need to be playable on Xbone so BC x360 games are an option. Any suggestion? 

Uh, Minecraft and Lego ... and uh .... yeah.

i think you will regret it imo

Peggle, which was just offered on GWG, is a fun puzzle game with local mp options.

Rare Replay has a bunch of old-school games that are kid-friendly.

Minecraft, of course.

Rare Replay, Plants vs Zombies, Lego Anything

Rayman Legends.

Besides LEGO and Minecraft, you should try looking at some indie games. Oh, Rare Replay too.

I played all the games ive wanted to on it and whenever i get the NX ill get the new Zelda. 

That was my exact logic when I sold off my GameCube.  Have regretted that numerous times.  Imo, unless you just hate the thing, never sell off a whole system+games.

As for the Xbox, Child of Light and Ori and the Blind Forest also have potential for appeal to children, though it's not a sure thing and they are both a bit melancholic.  But Mario and such?  I mean, there are options but honestly none that match Nintendo.  Maybe try Dust: An Elysian TaIL if it's available on Xbone?  

Yeaaaa. Xbox doesn't really cater to that demo too much. The Lego games are all good choices... If you have Kinect then there's Dance Central, Just Dance, and Kinectimals. The classic Sonics and Sonic Generations via 360 backwards compatibility are the closest you'll probably get to Mario. Rayman Origins and Legends are great, but hard. Minecraft is obvious. I wish you luck.