The other thing about this is the parent may assume you're ok with it. Presumably they have noticed that their daughter always comes back from lunch with you having a) had lunch and b) had money for a new toy as well. As the parents actually are giving the schoolmate money, I think it's fine to say 'Hey, this is a bit awkward but can I hold on to the lunch money this time? We're going past the toy shop and X has some formidable pester power. This way we can skip straight to using the lunch money for lunch."