No worries. I've had that message when I booked a party of 8. However, when I did book I didn't add the baby, 7 mos old and I called in a panic only to be told... not to bother showing up because they won't be able to sit you because the baby counts and there were no more openings.... at any of our restaurants I booked. Well, I worried myself silly.
Not once did anyone turn us away when I asked to add a highchair and the baby. Now, I did hear a party being turned away for adding an adult at Canada. I felt terrible for them because I didn't have an issue adding the baby/highchair.