No, it's not tacky at all to 'redesign' your response card. Since adult guests find themselves filling in the traditional "_____ will attend" section, or the ______ before each choice of chicken, salmon, or filet mignon, you can provide a wonderful service for both your guests and your caterer by adding a kids' response checkpoint below. It would read like this:
Kids' menu (note to guest: please fill in child's name and meal choice, which you provide)
So your card would have Mom and Dad's entries, plus the fact that Baby Charlie is going to be there and little Danielle wants chicken fingers. The caterer will hug you, and you get a clear picture of your guest list.
Remember, etiquette has bent to allow for just these kinds of unique situations, and no one's going to burn your response card because you formed your own model.

Most guests tell me they're very happy to see on a card like this that kids are included.
You can create a separate kids' response card in fun colors and with graphics, to make the little ones feel special and excited that they've been given a 'grownup' response card to fill out. It makes for a cute keepsake in the kids' handwriting too.