Very interesting information, Furball. Your grandparents resided near the site of "Little Bridge," the span that carried the Old Salem Road over Little Timber Creek. The swale formed by the road can still be seen from Lake Drive coming up from the creek.
Before the Hessians departed to attack Fort Mercer, American patriots had been out destroying bridges and removing boats upstream to keep them from enemy use. Since field intelligence told the Hessian troops that the American had already taken out the toll bridge over Newton Creek between South Camden and Gloucester, the Hessians left Coopers Ferry (Camden) and marched out Haddon Avenue to Haddonfield. From that point, they traveled down Kings Highway and in Market Street, Mount Ephraim/Gloucester, to reach the Little Bridge. Upon arriving on the north shore of Little Timber Creek, they crossed the bridge and moved down the Old Salem Road to Big Timber Creek, where they quickly discovered that the Americans had destroyed the bridge there. This forced the Hessians to double-back into Mount Ephraim and they likely crossed Little Timber Creek using Harrison's milldam, stretching between Mount Ephraim and modern-day Bellmawr. From there, the German troops headed out Creek Road/Browning Lane until they arrived at Warwick Road, which they used to arrive at Davis Road, which took the soldiers to the bridge over Big Timber Creek at what we call today Clement's Bridge. All of the extra marching explains why the Hessians did not arrive at the fort until very late in the afternoon.
Since the Hessians did not actually cross at the Little Bridge, the cannonballs found on your grandparents' property are more likely from British troops under the command of Lord Cornwallis. Cornwallis landed at the unfinished American fort at Billilngsport on 18 November 1777 with the intention of capturing Fort Mercer and taking control of Gloucester. Upon learning of the landing, Washington told the American forces to abandon the fort and destroy all usable goods not readily transportable. After learning of the destroyed fort, Cornwallis marched to the site of the Big Timber Creek Bridge, which the Americans had also taken up. At this point, his military engineers came forward and deployed a hollow copper portable bridge which allowed the British forces to cross the waterway. Cornwallis then marched to the site of Little Bridge and again deployed sections of the portable bridge to gain access to the north shore and Gloucester.
The cannonballs found likely either come from a defensive position established to protect the engineers deploying the bridge or, perhaps, from an upset wagon.
Sorry for the long message, folks!!
Best regards,
Jerseyman