Start with the part that is verifiable
Separate the event you can describe from the motive or reaction you are guessing. For a message about how to tell a co-parent about a school issue, use this factual base: state the child's schedule, health, school, expense, or behavior and distinguish observation from interpretation.
Make one answerable request
Give the recipient one concrete question or next step instead of several emotional jobs at once. 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.
Leave the recipient in control
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. A clear message remains useful even when the answer is no or no answer arrives.