How can you pay child support to your child and not his mother?

Child support is paid to the custodial parent. Generally you have no control over it unless you have evidence the mother is neglecting the children. In that case you would need to try to gain custody yourself or have the court arrange some supervision.

The problem of fathers who do not want to pay child support over to their ex-wives is extremely common. However, the custodial parent has the right to decide how to spend the child support in any way that benefits the children, within reason, without having to account to the child support obligor. It is not "spending money" for the child. Child support is provided for help in paying for the child's needs that can include anything that must be provided for the child including shoes and clothing, medical and dental expenses, school expenses, rent, recreational activities, heat and utilities and food.