My brother has his kids riding with him. They range in ages but the older kids are starting to come into their own and are starting to gain speed. They ride flats with some hills, do about 18-20 mph, and about 10 mph on small hills. They tend to ride about 30-50 miles at a time. He is interested in a hub or kit of some sort that he can add to his kids bike so he can keep up. He is about 3 mph off the others when in a proper train. My brother wants to try this to keep solo and not on a tag along set up.
So far I can only think of Ridekick Electric Powered Bike Cargo Trailer as an option. They ride with a sag wagon (the wife in a van) and this unit seems like a thing he could add on the hills. It looks like he may need an extra battery as well. Anyhow, any help would be nice!