Many of your guests will likely find the no-kids rule a hard pill to swallow. Yet, you can say it in several ways to make it go down better.
Soften it.
Soften your statement by letting your parent-invitees know that you adore or love their children but cannot accommodate them out of necessity. You can include their children’s names in your communication to show them that you value their kids. Make sure your tone is apologetic, conciliatory, and not at all off-putting.
Present it in a better light.
Through your words, let your would-be guests know that you also have their best interests in mind. Point out to them that the occasion can give a much-needed respite if they can just hang loose and enjoy themselves instead of worrying about their children on the loose.
Cite your specific reasons (yet be kind!).
In no way are you obligated to defend your decision to have an adults-only affair. However, giving your reasons can help your guests understand you and support your decision to go children-free at the event. However, note that it is not considered polite to say you are not partial to children.
You may need to cite other valid yet more acceptable reasons as it certainly isn’t your desire to offend anyone. It never hurts to be kind.