"The Lawrence Moeller Family of Pittsburgh PA"

Researching: MULLER, MOELLER, HILLENBRAND, HILDENBRANDT, HILDEBRAND, WETTER, WEBER, ASCHENBRUCKER, KLEINHERZ, TRABERT, SCHLOTT, SCHLOT, HAHN, ABEL, KRESS, TREMEL, BESCHORNER, FATH, FLEISCHEL, FAESSEL, HEIL, SCHAFER, SCHAEFER, STEIOFF, ZELIK, GAERTNER, EBNER, SCHREIBER, INSERRA, FLECKENSTEIN, VONDERAU, HERBERT, MATHES, HOHMANN, HOHMAN, KLINKNER, KLINGNER, STROBEL

My ggrandparents, Barbara HILDENBRANDT (HILDENBRAND) and Lorenz MULLER (Lawrence MOELLER)lived on Troy Hill, Pittsburgh, PA, in the late 1800's. Barbara emmigrated to America where she met Lorenz Muller, who had emigrated to the U.S. also. Both were from Germany. They married Nov. 17, 1863, in St. Mary's Catholic Church located in Allegheny City (which was incorporated in 1907 into the City of Pittsburgh, PA.) They had 12 children: 1) Elisabeth; 2) Joseph; 3) Christian; 4) Emma; 5)Katherina "Kate"; 6) Maria Eva; 7) Bernard John; 8) Lawrence Sylvester; 9) August George; 10)Peter Joseph; 11) Margaret; 12) George Joseph.

Most of these children were buried in St. Mary's Cemetery on the North Side of Pittsburgh and St. Joseph Cemetery in Bellevue, a suburb of Pittsburgh.
I am trying to locate the burial places of 3 children who were born and died in Allegheny City, PA:
1) Elisabeth Moeller (born Sept. 14, 1864; died Oct. 6,1868; --age 4).
2) Joseph Moeller (born Mar. 1, 1866; died Dec. 4, 1874--age 8).
6) Maria Eva Moeller (born May 14, 1873; died Dec. 23, 1874--age 1)
I have checked the cemeteries where their parents and siblings are buried. The cemetery records do not carry their names.
If anyone associated with cemeteries can advise me where to look next, I would appreciate the guidance.