Here is the Wikipedia account, which matches pretty well with what the old gentlemen told me. I think their grandfathers had been there during that time.
During the mid-1864 Paw Paw Rebellion in north-western Missouri, detachments of Federal troops crossed the Missouri River and occupied Platte County, Missouri. At this time a Confederate cavalry force approximately 200-300 strong under Colonel J. C. Thornton was organizing around Camden Point. On July 13, Thornton's men held a picnic in an open pasture near the town. Detachments of the 2nd Colorado Cavalry and 15th Kansas Cavalry, totaling 700 - 1,000 soldiers, ambushed Thornton's picnicking force, routing it and killing two and wounding approximately 25 Confederates, having suffered one killed and fourteen wounded. Three of these wounded Federals later died from their wounds. Four additional Confederates were captured and executed by Federals after the battle ended. Ammunition, weapons, and gunpowder were captured and Camden Point was burned.
The battle flag of the Confederate force was captured as well, and now resides in the possession of the Colorado State Historical Society. In 1871, a memorial to the Confederates killed in the engagement was erected at the Pleasant Grove Cemetery near Camden Point where the Confederate slain are buried and is the third oldest Confederate memorial west of the Mississippi River. Two older Confederate memorials can be found in Lone Jack, Missouri and Chowen Cemetery (Wayne County, Missouri) erected in 1870.