You're definitely in quite a predicament! I think convincing may be a bit easier than you'd think, especially if they care for their guinea pigs properly. If it were me, I would try emailing them regarding the matter, this is better than in person as most people don't tend to take children (or most juveniles for that matter) seriously. The anonymity of an email can give you the benefit of at least getting them to hear you out! I would politely explain why cleaning hamster's out too often does not provide any benefits at all, but rather makes them more stressed since their scent has been removed. You can then explain that stress (just as in guinea pigs) can lead to illnesses. As for the cage thing, there are lots of wonderful studies that have been conducted which prove that hamsters, even more than most other rodents, need huge cages in order to be healthy both physically and psychologically. I would talk to them about how a larger cage generally=less stress which means less illness and other issues. You could also link them to up to date care guides, studies, and even this forum if they need extra resources! Good luck with this, please let us know how it goes
