How woould you respond to an RSVP card that you got back asking a question?

So I got an RSVP card back for my wedding which is in the beginning of May. On some of our invitations we put, ‘we regret that we will not be able to accomodate babies and children.’ On an RSVP card that I got back, it listed the couple’s name and the child. At the bottom it had an asterik that said, “If our daughter is too young to come, we regret that we will not be able to attend” First, the daughter is 6 and we’re not having kids at the wedding except for our immediate family. What is the proper way to respond back basically saying, sorry she’s too young, sorry you’re going to miss a good time…I have no idea how to react to that…