The usual reason for a device heating up is 'overwork', just as it is for a person heating up. Some games (eg. those with lots of action involving high-definition graphics) put the processor through a heavy workout.
Apps that have been hidden, but not quit, add to the load as they continue to work away in the background.
Keep track of the games that cause this overheating. Quit any process (app) you are not currently using. Remove any files you will not have need for. Then see if there is any improvement.
Note that this is general advice, applicable to whatever platform you are using.
You've posted your question in the AppleWorks community, where the main topic of discussion is issues and techniques involving Apple's long-discontinued productivity application AppleWorks. You may receive more specific advice in the Using iPhone community, so I've requested this discussion be transferred there.