If there are more than 2 years between children, maybe one died. If there is a large gap between children there might have been a prior spouse or blended family. In my family, the 13 children listed with my great grandparents in the early 1930’s were not all theirs. The Great Depression was difficult for everyone and during those years the city relatives sent the children to the “farm.” This insured that there would be food for them and helped the farm because they could not afford to hire help. The parents may have split up temporarily, as my ancestors did, in order to find work or reduce expenses. So don’t assume that all children in the house are biological children.
I’ve also found that once a child reaches puberty, they are often listed as “boarders” instead of sons and daughters. I’ve also seen a 13 yr old girl listed at the bottom of a census family and she is not a daughter but a servant. Check the other households in the neighborhood for servants, also. Neighborhoods tend to be homogeneous – the old “keeping up with the Joneses.” If one family has a servant, it is likely that others on the same street will, also. So be careful when looking at these records and don’t apply 21st century family structures to 1850 households.