Make the first sentence usable
Lead with the reason for writing so the recipient does not have to decode a long emotional introduction. For a message about how to agree on discipline with a co-parent, use this factual base: state the child's schedule, health, school, expense, or behavior and distinguish observation from interpretation.
Reduce the work of responding
Offer simple options, a narrow question, or a clear update that can be understood without another conversation. 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.
Respect the boundary already visible
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 block, explicit refusal, legal restriction, or safety plan takes priority over the desire to explain more.