Why can’t I just give the money to my child?

Hi,in short i over paid when i first started paying the csa due to a error regarding the csa didnt take my mortage in to account on how much i should pay a month.

I paid 320 for 18 months until the csa told me it was too much,now 12 years later i now need to pay 581.60 in arrears.

How or why after 12 years i have to pay this not to my son ,but to his mother.no on at all would rather give this my son.