I have two kids. We agreed that he would pay child support and he has done that for the past 3years we live in different states. I have primary physical custody and we have shared legal custody that our lawyers wrote up and signed and we agreed that they would live with their dad for high school they just completed 1yr of high school. We signed and agreement that he could never get child support from me because I have always supported the kids financially without his help up until the last 3 years. The child support also had suspended time for example in the summers but now I will get them in the summer. Dealing with interstate courts they have messed up our arrangement multiple times making it a hardship. I have written a letter to cancel the child support and that he and I both agree we will support our kids financially without the assistance of the courts in either state and we will not take either parent to court during the time our kids are under 18yrs old living with either parent and we both signed it and getting it notarized. Will this agreement hold up as a contract between two parents? Or do I risk my kids father taking me to court for child support despite our agreement?

You need to get the agreement ratified by a court. Where was the original order issued and does he still live there?