What should a girl do who, as a child, stole her cousin's dolls and gifted them to her friends, and now feels ashamed to tell him or to tell her father?

There is no sin upon one who stole before puberty, but he must return what he stole, or its equivalent, or its value, because of the Prophet’s (peace and blessings be upon him) saying: “On the hand is what it has taken until it fulfills it.” If shyness prevents him from disclosing it to the rightful owner, he can deliver the amount through stratagem or insinuation.