My son found 100 shekels on the pavement.
Besides the large amount of money there were no simanim.
Is there a mitzvah of hashavos aveida?
There is no obligation to return the money to its owner, and the mitzvah of hashavas aveidah does not apply.
However, if the owner is known it is correct (lifnim mi-shiuras ha-din) to return it to him, and this will be a good chinuch for the child.
If the owner is not known, finding him will be problematic, unless you declare the loss and assume that the claimer is telling the truth. There is no obligation, even lifnim mi-shiuras ha-din, to do this.