I may have found John as a child! In the 1870 Census for 5th Ward, Quincy, Illinois I found a David Galloupe with wife Kate A. (both born in PA) and children John (12-so the birth year fits!), George C. (8), Mary E. (6), Benjamin (3), and Anna B. (1). First three kids born in MO, last 2 in IL.
I found this same David as a 17y old in the 1850 census for Montgomery Co., IL with parents Benjamin & Sally (both born in NY) & children all born in PA: Benjamin (20), then David, Syntha (15) and James (5)
There is a second Galloup family list above them that is probably father Benjamin's brother (they're 2 years apart):
John (54), Margarett (19), George (2), and John (6M).