Even a newborn baby in arms counts as they'd be included because of the fire codes. Due to humongous fines, WDW restaurants are not going to go risk going against the fire codes. If it's breathing, it counts toward the fire codes. Plus as you said, a 20 month old will be taking up a seat, so they would need a 6 topper instead of a 4 topper (WDW doesn't have 5 topper tables).