Is one absolved of responsibility by returning stolen money discreetly, despite the fear that its owner may not become aware of it, and does the ruling differ if the theft occurred before puberty, knowing that charitable donations have been given with the intention for the owners of the money?

Stolen money must be returned to its owners, and donating it does not suffice as long as the ability to return it is available. As for the money you left with your relative, you are not obligated to inform him that you stole it. Rather, the obligation is to deliver it to him in any way that makes it enter his possession and absolves your liability, such as sending an anonymous message stating that the money is rightfully his.