According to the 2000 Census, the average US family size was 3.14 people.
The average number of children per all families was only .90 child.
The average number of children in families with children was 1.86 children.
So families with children tended toward 2 children, but not significantly if you allow for the number of families with MORE than 2. The number of families with 1 is likely close to the number of families with 2. More than half (52%) of ALL families had no minor children at all.
With changing demographics, this number could be larger in the 2010 Census. Larger families would weight both averages upward.