Based on this data, none of these individuals could have been involved in the Spanish American War. Contrary to popular belief, underage service was quite rare (with the rare cases being 16/17 years of age) in the Spanish American War. Competition for a space within a military unit was intense. Those who were not qualified were quickly discovered and replaced with individuals who were qualified. Drummer boys were not boys, but men charged with the very serious duty of military communication. Stable hands were adults also.
Given the dates of death, the U.S. was not yet involved in World War One until 1917, so Moffatt would have had to have joined the regular army serving in the U.S. or he would have had to have joined a foreign military unit.
You may want to look in the newspapers of the day from the area in which they were from to see what other epidemics were passing through the area at the time, such as possibly diptheria, etc.