When the family is riding in the taxi chasing after Stuart, the baby girl is securely strapped into an infant carrier. When they climb out of the taxi, the baby girl is in the mother's arms and the infant carrier is nowhere to be seen.
Toward the beginning of the film when Stuart flies the model plane outside and crashes it, his mother, father, brother, and friend all run out the house and into the park to chase after him. This means the baby was, presumably, left alone in the house - or, the writers just forgot about her completely.
When the family is riding around in the taxi looking for Stuart, the baby girl bounces back and forth several times between sleeping and being awake. It appears to be the same shot of her every time you see her sleeping.
When Falcon is arguing with the yellow bird, they are atop the Chrysler Building according to the eagle decals, but when they swoop down, they are next to Central Park. The two are nowhere near each other.