Does a baby putting one of his legs over the other indicate the coming of a new baby to the family, or is this a false belief?

This matter is a superstition and a claim of knowledge of the unseen. Any prediction or claim of knowledge of the unseen, which Allah alone possesses, is an outright lie that should be disregarded. There is no connection between a child placing one leg over the other and what will be born to their father. Knowing what will be born to a person is part of the future earnings that only Allah knows, as affirmed by His saying: "Indeed, Allah [alone] has knowledge of the Hour and sends down the rain and knows what is in the wombs. And no soul perceives what it will earn tomorrow, and no soul perceives in what land it will die. Indeed, Allah is Knowing and Acquainted."