How to address save the dates for a child free wedding?

If you don't want children there you would normally add this on to the invitations rather than saves the dates. You can make it light hearted by saying something along the lines of 'In order to allow all guests, including parents, an evening of relaxation we have chosen for our wedding day to be an adult-only occasion. We hope this advance notice means you are still able to share our big day and will enjoy having the evening off!' or 'We would love to give all our guests the opportunity to let their hair down and have a good time without having to worry about little eyes and ears so we politely request no children.'

Most of the time it would go on the invites, but I guess if you want to give people the chance to find childcare for the night/weekend then you could put "Book the Babysitter and Save the Date" - This is probably what we are going to do.