Recognize what makes this moment difficult
Acknowledge the event directly without turning it into a lesson, comparison, prediction, or silver lining. For a message about how to update a co-parent about a medical issue, use this factual base: state the child's schedule, health, school, expense, or behavior and distinguish observation from interpretation.
Offer support that can be accepted
Offer one specific form of practical or emotional support and make clear that no immediate reply is needed. 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.
Follow their pace rather than yours
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. The recipient decides whether they want conversation, ordinary company, practical help, professional support, or quiet.