I would put a note in with the invite. What you have put is fine but personally I don't like the joke. I don't see time away from my children as freedom - if I wanted that I would not have had children, and personally I like going to big occasions with my children.
As for the babies I would put something on the invites about it but remember depending on the age they may need a highchair (6 months plus) also remember that babies come with rather large pushchairs which will need placing by the tables near to the parents.
I personally would word it
Unfortunately, due to being very limited on numbers, we have had to decide to only invite babies under 12 (or change number) months and family children to our wedding. We hope you understand and are able to celebrate with us on our Big Day.
I would leave it like that and put no joke in.
Remember though when thinking about the age of babies that some 8/9 month old babies are still breastfed so mum will not be able to leave them all day if this is the case. I know even at 10 months I would have been unable to leave my child for the whole day.