What is the expiation for killing small ants (al-dharr) with hot water by someone ignorant and young in age?

It is forbidden to kill ants unless they are harmful and cause injury, in which case it is permissible to kill them. It is not permissible to kill ants with fire or hot water, because the Prophet, may Allah bless him and grant him peace, forbade torture by fire. There is no expiation for one who kills ants with hot water except repentance, and a minor before puberty is not held accountable.