I think that the problem in general with surveys that calculate averages is that an “average figure” is not necessarily the same as “typical” or “most”, and is not necessarily a good indication of “typical” or “most”.
For example, the average salary in a country does not necessarily reflect the amount of money that most people earn, since there may be a minority of people who are earning way more than the average, and are therefore significantly raising the average.
I think the median is generally a better way to figure out what is typical than the average is.
When it comes to family size, if for example, out of 10 families, 7 of them have 7 kids each, but one is still single, one can’t have kids or doesn’t have kids yet, and one only has one kid, the average will be 5 but the median is 7.
I think that the median is probably a better way to figure out what is typical than the average.