Do you have to pay child support if you are remarried and are a stay at home mom?

Ultimately this has a very simple answer. Whether or not any person has to pay child support is determined by a court order, usually in the form of a divorce or custody decree. If that says you have to pay child support of $X per month, then you have to pay child support of $X per month.

If your circumstances have changed since the court order was issued, you can go back to court and petition to have it changed. Until it is changed, you'd better keep paying. The court may also decline to modify the decree under some circumstances.

If you're a "stay at home mom" by choice, then presumably your family income is high enough to support that lifestyle and your obligation to provide for your older child(ren) is still fully in force. You don't get to cheat the children you had with your ex out of the support you owe them.