Maggie

This movie is all about subconscious needs. Mom needs to be closer to her husband, so she "unwittingly" creates an invisible child so that Dad will be more attentive. Dad wants to take care of Mom and not lose her to an institution, and probably feels unacknowledged guilt about being too busy, so he plays along, hoping she will come to terms with this fallacy. Since that never usually works, since they are both getting what they want/need, even if it's unhealthy, the nanny comes in to break up the cycle. Mom is slowly able to come to terms with it in a non-damaging way, and things progress. The oldest child (in the usual role assigned to them) gets to take care of everyone. The youngest child teams up with Mom since he's the baby, and probably doesn't understand yet what's actually going on, so to him its just imagination a lot of time.

It's also a good example of how outside people see something odd ( like Mom playing with no-one) and mind their own business or just look away. Except for the restroom lady. : >