This appeal concerns a dispute between Samuel and Susan Gaston over child visitation privileges. The Gastons signed a custody agreement that allowed them to change the physical custody arrangement after one year. The custody agreement also provided that if the Gastons disagreed on proposed changes, they could seek mediation or court modification. Nine months after they signed the agreement, the Gastons adopted a slightly modified visitation schedule. Approximately one year later, Susan decided that the new arrangement was not in the children's best interests and informed Samuel that she wanted to return to the original schedule. Because Samuel wanted to retain the new visitation schedule, he filed a motion in the superior court requesting mediation or court modification, as provided for in the custody agreement. The superior court denied Samuel's request, ruling in part that the agreement's mediation provision had lapsed. Because we conclude that Susan was estopped from arguing that the provision had lapsed, we reverse.