Q: Is it ok to allow a child to give money (voluntarily) from their own money as sadaqah/ towards a gift?
A: A child cannot give his money in sadaqah. An elder may give the child some wealth for the child to give in sadaqah. This is permissible. In this situation the child will be passing on the wealth of his elder to the poor.
Allah Ta’ala (الله تعالى) knows best.