Yes, I would explain it to the parents that this is not tolerable and that they need to wash it thoroughly and keep it away from the kittens!! I have had cats all of my life and in the instance that you had an un-neutered male cat in the house, you would likely have a problem of them spraying around the backpack as well!

That is just gross and any normal person would have the courtesy to do something about the smell before bringing it in your house.....I would be so embarrassed if I did that to someone else!!