Name the impact without exaggerating it
Describe who had to wait, worry, redo work, spend money, or make a choice with incomplete information. For a message about how to address an overdue child support payment, use this factual base: state the child's schedule, health, school, expense, or behavior and distinguish observation from interpretation.
Make repair practical
Offer the repair you can actually complete, with ownership, timing, and any limit stated honestly. 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.
Let future behavior carry the proof
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 promise inside a message is weaker than a visible change in what you do after it.