If a parent is actively interfering with your court-ordered visitation, involving law enforcement at the time it happens can create an official record that helps later in court.
Bring a certified copy of your custody order when you contact police, and ask an officer to accompany you or meet you at the exchange location. An officer who sees your order and witnesses a denial can document it in an incident report, which carries real weight in a later enforcement hearing.
Every state handles custodial interference differently — look up your own state’s statute (see our State Family Codes page) and make sure your underlying custody order has specific, enforceable language (exact days, times, and locations) rather than vague terms like “reasonable visitation,” which are much harder to enforce.