You assume they'll be less focus on whatever they're doing now. Fact is they probably hire a nanny to take care of the kid, i.e. more taxes for the government (assuming nanny does report taxes). Plus anything spent on the new kid is GST income accretive. More money for the gov.

Moreover, your model is pretty one-dimensional and doesn't take into account various factors such as satisfaction/better quality of life and thus greater productivity as a result of having a kid.

But i like your crude/offensive/realistic efficiency thought.