Thanks to everyone for he ideas/alternative explanations.
a few more details - this was isolated to the 1930 census.
It was the inventory of the house my Grandparents lived in for many years and the list included my Grandmother and Grandfather, my mother one aunt and one uncle all the lived long lives.
There is no question about them being my family, confusion of the name, etc.
The mystery entries are three younger children, listed in the census as children.
None of my family have ever heard of temporary cousins, hidden neighbors, child mortality at this time.
There were lots of stories about earlier infant mortality in Russia so children dying was not a tabu subject in the family.
Only three possibilities I can imagine: 1) The Census taker made up the extra children for some reason, 2) My grandparents invented the extra children for some reason, 3) Some neighbor answered the census for my family incorrectly.
This was the depression - would there have been a reason for making your family appear larger?
HERSKOWITZ, HERSHKOWITZ, BOROFSKY