What is the ruling on leaving the Friday prayer and then returning while the preacher is still delivering the sermon, and is the Friday prayer considered valid in this case?

There is no harm in you leaving the mosque while the Imam is delivering the sermon to take your child home if there is a need for it, such as if the boy's remaining in the mosque would cause disturbance to the worshippers or harm to him. None of that affects the validity of your prayer.