She allowed her son to go the mosque and he drowned in a hole in the valley and died

My son who was five years old has passed away. He drowned in a hole in a close valley to the village in which we live. We did not know that he went with friends to the valley to play. It is a habit for them to go after asr prayer to play, moving between their grand parents’ homes. We are a big family and our homes are close to each other. We left to pray in the masjid and I left him with his mother at home. He insisted on going to pray with us saying “I do not want to enter hell, I seek paradise, please let me go to pray” so his mother felt for him and let him go. He returned dead! Are we “his mother and I” sinful? His mother is very sad and wants to know if she is sinful or if she needs to expiate.