Yes, because they will need room to place the high chair.
Say you are a party of 5 with the baby. If you don't count her, they will place you at a table for 4. Then you show up and they have to move you to a large table to accommodate the high chair and then they have to displace someone else to a smaller table. Or they may have to leave you at a more crowded table because there are no larger tables available.