Photos from my iCloud ending up on my son’s iCloud

To start off this was my mothers phone. She got a new one. So she lets my son use her old one on WiFi. We had some issues with him buying things he shouldn’t have. In the beginning, it was all her photos on there because it was under her iCloud account. So I decided to make him his own iCloud account but because he’s so young, it’s within a family plan thing. But it’s always been separate iCloud accounts. I don’t remember authorizing my photos to be on his phone. But now instead of her photos, all my photos are on there: any ideas? We all three have separate iCloud accounts. And only me and him have the family plan thing. How did her photos disappear? And how did mine end up on his phone? None of his photos that he’s taken since have ended up on my phone, thankfully (it’s all pictures of logos and kid games). Realistically, shouldn’t he start with no photos at all unless he takes them because he’s starting with a brand new iCloud account? Thank you so much. P.s. he’s still only using WiFi and doesn’t have a phone plan.