Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to request payment for a shared child expense, use this factual base: state the child's schedule, health, school, expense, or behavior and distinguish observation from interpretation.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Propose a child-focused plan, a response date, and the documented route for unresolved decisions. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not use the child as messenger, trade access for agreement, or criticize the other parent to the child. Follow custody orders and use safeguarding, legal, medical, or emergency support for risk to a child. You do not need the other person to agree with a boundary for you to follow it consistently.