It is interesting to think that the latter two children could have been children of Philip and Elizabeth. If she was born in 1790, then Elizabeth would have been 50 and 55 when the last two were born. I found this same problem when I was researching my other side of my family. By the way, Philip and Elizabeth were my gr-gr-gr-great grandparents.
Although I know it isn't impossible for a woman to give birth at age 50, I do know that she would have probably gone through menopause by age 55. I am wondering if one of their children (sons perhaps) might have lost his wife and left the kids in the care of the grandparents? That seems to be what happened in my search of my mother's family.
Just some thoughts.
Gail Surrena, Wasilla, AK (formerly Western PA too)