Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to tell a date you have children, use this factual base: refer to what actually happened, what you want now, and any personal information relevant to informed consent.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Make one clear invitation or boundary and leave room for a different pace or a no. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not flood someone with messages, demand exclusivity, or use vulnerability to create an obligation. Meet safely, protect personal information, and stop contact when a refusal, block, threat, or deceptive identity is involved. You do not need the other person to agree with a boundary for you to follow it consistently.