First and foremost, every parent has a duty to support their children. Once parents split up, each parent's obligations and rights become a bit tricky. In the majority of cases, one parent is granted custody and the other is ordered to pay child support.
The noncustodial parent is required to make timely payments each month pursuant to the terms of the court order. Even if the parties agree otherwise, the terms of court order must be followed. The parties' agreement does not modify the court order!
A lot of times, the noncustodial parent will decide to chip in and pay extra for sports equipment, dance lessons, or a cell phone. However, this does not give the noncustodial parent the right to stop and resume payments at his/her leisure.
In Virginia, "overpayments" are considered "gifts" and are typically not credited toward missed payments. When payments become inconsistent or completely stop all together, the custodial parent may file a show cause (contempt) due to the non-conformity with the court order. The noncustodial parent is free to pay as much as he/she desires but, the underlying obligation as required by the court order must be met each month.
On the other hand, the custodial parent has no grounds for a show cause if the noncustodial parent is simply violating the parties' agreement but not the court order. In other words, if the parties agree to a greater amount of child support, and the noncustodial parent later decides to pay only what is required by the order, the custodial parent has no grounds for contempt. There may be grounds for a breach of contract but, not contempt. Again, the terms of the order remain the same regardless of the parties' agreement.
So, while it is great when parents work together for the benefit of their children, a court order cannot be modified without a judge's approval.
Therefore, always follow at least the terms of the court order!