Hi,
Mine are 5 and 7. When they were 2 and 4, we had the Schwinn Trailblazer double trailer. A bonus for the trailer is that it has loads of storage - we used to bring water, snacks, jackets, wipes, music, etc. After my little one turned 4 (and the older one was riding on her own), we switched her to a Schwinn Runabout Bicycle Trailer. Not specific affinity to Schwinn - that's just what I had found on CL that fit the bill at the time. Very happy with both purchases.
As to best trailer, that would depend on your needs - there's cheep, fast, light, durable, portable, go-anywhere, etc. - what are your priorities?
When they were babies, they had the iBert Child Bicycle Safe-T-Seat that's mounted on the steering column - they enjoyed it but outgrew fast. I was never a fan of the rear mounted seats because of the "view" the clid gets
.
Good luck!