Write from facts rather than momentum
Pause long enough to remove accusations, assumptions, and details that belong to someone else. For a message about how to discuss introducing a new partner to children, use this factual base: state the child's schedule, health, school, expense, or behavior and distinguish observation from interpretation.
Offer a proportionate next step
Match the size of the request to the relationship and to what the recipient can reasonably decide now. 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.
Protect privacy on both sides
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. Use the most private legitimate channel available and do not share the exchange to recruit support or pressure.