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I think your hams are Russian dwarfs aren't they?
Balls can be particularly unsafe for dwarf hams, they are more likely to get injured by banging into things or to get their feet trapped than syrians are & they are often quite scared by them which is a problem as you have no way of knowing if they are running for fun or in a desperate bid to escape!
Dwarf hams are a whole lot easier than syrians when it comes to play pens too, you can buy metal ones & they can't climb out or you can make one quite simply.
Playpens are also a lot more fun for both ham & owner if you put toys & other things to explore in there & it makes interaction with your ham or hams a whole lot easier too.

Opinions do differ but mine is for dwarf hams at least the only thing a ball is good for is transporting from the cage, otherwise I would bin them!