I checked the 1850 Census Record in Dennis Township, Cape May County, NJ. There were two FOSTER Head of households during that year. One being a JOSEPH FOSTER; the other a URIAH FOSTER. Uriah is most likely not the one, as he would've been only age 15 when your REUBEN FOSTER was born. So it could be JOSEPH FOSTER.
Looking at the names of your REUBEN & CATHERINE's children, I see one given name the same in JOSEPH FOSTER's children; that of ALEXANDER. Also your RUEBEN & CATHERINE named a son, JOSEPH; possibly after his father.
You may be able to find a will for JOSEPH FOSTER, b. about 1883 in NJ. That may give you the children's given names, which may include the name of REUBEN, and possibly name some of the grandchildren, as well.
July 26, 1850 Census: Dennis Township, Cape May County, New Jersey.
FOSTER, JOSEPH, age 67, (Could not read occupation), b. NJ.
FOSTER, ELLEN, age 53, b. NJ.
FOSTER, LYDIA, age 28, b. NJ.
FOSTER, CONSTANT, age 24, Occupation: Mariner, b. NJ.
FOSTER, HARVEY, age 19, Occupation: Mariner, b. NJ.
FOSTER, ALEXANDER, age 14, b. NJ.
If you find that this is the correct family that your REUBEN FOSTER was born into; you may be able to get the death certificate for for JOSEPH FOSTER, who died June 9, 1868 in South Dennis, Cape May County, NJ. A death certificate can be gotten form the NJ Archives located in Trenton, NJ. You may be able to do so by phone. An obituary may be gotten also. You can check with the Society below for that. They have the old newspapers on microfilm in their research library; although many times, not much info. is given in them, unless the person is of some importance.
You can check with the Cape May County Historical & Genealogical Society in Cape May Court House, NJ. They have a website on-line. The person you could ask for would be Lois Broomell.
Hope this can help you some.