Will Allah Almighty forgive one who hurt his mother in his childhood and regretted it after her death, and is there any expiation for this deed?

What you did to your mother when you were ten years old is not held against you, and there is no sin upon you for it. This is because the pen is lifted from a child until he reaches puberty, as the Prophet (peace be upon him) said: "The pen is lifted from three: from the sleeper until he awakens, from the child until he reaches puberty, and from the insane person until he regains his sanity." It is incumbent upon one whose parent has passed away to strive to do good to them and show piety towards them after their death, and this includes supplicating for them. This is based on the Prophet's (peace be upon him) saying: "When a person dies, all his deeds cease except for three: a continuing charity, knowledge from which others benefit, or a righteous child who supplicates for him."