No, it depends on the distribution. Let's say (as a first guess) that number of children follow a poisson distribution. A poisson distribution with mean of 1.84 has 74% of children being first borns.

There are a lot of reasons why a poisson model probably isn't great, but it's worth pointing out that a model with more density near zero is probably likely and implies that first borns are more popular than the they look.

74% versus 80% isn't anything to write home about.

Moreover, as Mz points out, only-children are a totally different case.