What is the ruling on an amount of 50 riyals found in a mosque, and is it permissible to donate it on behalf of its owner?

If the amount in Riyals is small, such that people of average means would not bother to pursue it, then it is permissible for the boy to own it, and you must give it to him. However, if it is an amount that must be announced, then the boy's guardian must announce it for a year, and then it enters the boy's possession after the year. It is not permissible for you to give it away as charity in either case; rather, it must be given to the boy's guardian.