Here are the results of my research into Caldweel/Caldwell in Boulder county and the surrounding area.
First, I personally walked both the Niwot Cemetery in Niwot, Colorado and the Burlington Cemetery in Longmont, Colorado.There were no Caldweel/Caldwells buried in either one.
Second, I did some research at two libraries and here are all of the Caldweel/Caldwells that I could find.
Mrs. Sarah Caldwell died September 4, 1899 near Longmont, Age 74BOULDER NEWS4 January 1900:1
Mrs. W. J. Caldwell died May 4, 1881 in Malta, near Leadville, Colorado.She was formerly of Boulder BOULDER COUNTY HERALD WEEKLY11 May 1881:1
W. L. Caldwell was mentioned in the following: Prof. Byron C. Smith died May 3, 1877 at the home of a relative, W. L. Caldwell, in Boulder of consumption.Age 27.Formerly of Kansas State University.Mother arrived three weeks ago.She took body back to Humboldt, Kansas for burial.He was the son of G. P. Smith, Esq. BOULDER COUNTY NEWS11 May 1877:3 DENVER WEEKLY TIMES16 May 1877:2
Mt. Pleasant Cemetery in Erie, Colorado had the following: Caldwell, Jacklynn3 months Alfred S. 13 months Samuel R.1880-1941 *Elizabeth M.1909-1978 b. March 4, 1909 d. February 22, 1978 in Denver *Alfred W.1905-1970 b. September 14, 1905 d. July 1, 1970 in Denver *On same headstone
Home of Peace Cemetery in Grover, Colorado had the following: Caldwell, I. Bertram 1883-1932 James W."Father" b. August 7, 1841 d. September 22, 1914 Nancy J."Mother""Wife" b. March 1, 1848 d. March 19, 1913 Max Manford b. September 24, 1916 d. September 12, 1917 William R. b. February 2, 1873 d. August 26, 1925
Eaton Cemetery in Eaton, Colorado had the following: Caldwell, Annabelle Hogue1873-1959 d. March 2, 1959 in Pueblo Hogue, Howard Eugene1865-1930
In addition to the above transcribed cemeteries, I also looked in these cemeteries but without any luck: St. Vrain Church of the Brethern in Hygiene, Colorado Columbia Cemetery and Green Mountain Cemetery both in Boulder, Colorado Caribou Cemetery in Caribou, Colorado Jonas Barber Cemetery in Eldorado Springs, Colorado Fremont's Men Cemetery in Deadman's Gulch Faivre Ranch Cemetery Gold Hill Cemetery Jamestown Cemetery in Jamestown, Colorado Lyons Cemetery in Lyons, Colorado Nederland Cemetery in Nederland, Colorado Ryssby Cemetery Rowena Cemetery Salina Cemetery Sugar Loaf Cemetery Sunshine Cemetery Sunset Cemetery South Boulder Cemetery in Boulder, Colorado Valmont Cemetery
Since you never told me when you think George William and Gnevieve Caldweel/Caldwell died, I don't know if they would have been dead at the time that these transcriptions were made.Hopefully some of the above names will be in your family and will give you new leads.