I think child support should be based upon the earnings of both parents. Naturally the custodial parent whether that be mom or dad should receive financial assistance to cover the additional costs involved providing meals, clothing, and other needs.
Nevertheless the goal is not to impoverish or "punish" the non-custodial parent. If a man for example is making $10 per hour and works 40 hours a week grossing (before) taxes $400 it's unrealistic to expect the court to force him to pay half of his salary making it impossible for him to have a decent place for his children to visit him as well as provide for his own day to day living expenses. The less a man earns the less money he will be required to pay in child support.
As for mom working 25 hours most women work full-time and collect child support. The court is not going to punish her for having a full-time job if she is the custodial parent. However courts have been known to award custodial parents more child support money if the ex increases his or her income significantly. In some instances they will take into account the income of someone whom he/she marries.
On a related matter a lot of men are upset about not being able to collect refunds for child support payments they made when it is revealed that they aren't the biological father! This is a combination of fraud and betrayal! Imagine paying 18 years of child support for a child that is not yours! It happens and it's not fair.
Maybe should have automatic DNA testing with every birth.The state however would rather "assume" the husband is the father than to risk having to assist the mother financially. They apply what is called "the best interest for the child" law if two or more years have passed prior to the discovery the man is not the father. The mother is never punished for her deceit. In their eyes the child is attached to him. No woman would want this to happen to their adult son or brother.