id personally probably be so embarrassed i wouldn't even bring it up.
I completely get forgetting to cash checks. Ive gone until the following payday meaning the next day the parents were supposed to pay tuition and have realized i haven't even made it to the bank.
Now i mobile deposit my checks and file them(in case it doesn't deposit right) as soon as i get them so I don't do this anymore.
but if i did for some reason really want to go about it, I would tell the parents it was my bad, I did forget to deposit it a few months back and at that point if the parent is willing to PAY for my mistake meaning they want me to cash it now then id do so still feeling bad. Or if they didnt id just apologize again and make sure that didnt happen again
And I wouldn't be too quick to think they noticed the money not taken out though that very well may be true. I know a lot of younger families that dont balance their accounts regularly. Me being one of them. because everything is digital. I may scroll through my banks site every once in a while for a detail I'm looking for but for the most part we unless something looks really off we don't go searching for each payment and statement.