Can you give me an idea about children being forced to kill each other off in an arena or closed area that doesn’t rip off Hunger Games?

Put all of the self proclaimed "authors" who ask questions here in an arena and promise that the last one standing will get 10,000 views on Wattpad. That'd work.  

Children get bussed to an island and they have to kill each other off for a TV show. This is the plot of Battle Royale which the writer of Hunger Games insists she didn't copy.

If you already thought it would rip off Hunger Games, then clearly it does.

Go ahead and write it how you like. The Lord of the Flies theme is Joseph Campbell level universal at this point. Battle Royale is the Japanese version, so what? Suzanne Collins doesn't own the trope, anymore than the estate of Ian Fleming owns the suave secret agent trope. She obviously got the trope from William Golding in the first place. 

If you already thought it would rip off Hunger Games, then clearly it does. 

The competition could be something which didn't explicitly require them to kill each other. They might even go to some lengths to hide what they're doing. Different alliances could form with disinformation going around about what the other teams intend and even about the rules of the game. Maybe the idea that there's a competition at all could be a bit suspect. What if some people are getting messages giving them instructions for a competition and others are unknowing participants? People getting messages could disagree about what they mean and who they're coming from. They might try to fraud versions of the messages to misdirect each other

No real writer ever has to ask other people for ideas.

And any story like that is obviously going to be a rip-off of The Hunger Games.

Anything along this line will seem like a rip off. 

Can you give me an idea about STORY CONCEPT X that doesn’t rip off BESTSELLING SERIES USING STORY CONCEPT X?

--> Can you see why your question doesn't compute?

I could point you to books that didn't rip off Collins, e.g. Takami Hiroharu's "Battle Royale", but that is primarily because they written were BEFORE.

But now, any book using HG's central concept as central concept too will inevitably be compared to that series. And frankly, why shouldn't it?

Variations on a theme.

Sounds like you already know that it WOULD rip off Hunger Games.