My son just out grew his over the winter. Trailercycles help teach kids where to ride in the road, how a balanced bike should feel, road manners, and bits of bike maintence.
Use a MTB or hybrid comfort bike with good low gearing. Carry a spare tube for the trailercycle and a 15mm open end wrench, as it doesn't have a quick release. I used a small inexpensive seatbag on the child's saddle for the tube. After fumbling with water bottles, I bought my son a Camelback Skeeter hydration pack. The C'back had the added bonus of an eye catching orange panel.
If your using an MTB, find some nice, easy trails out away from the road. That's an experence you'll never be able to replicate once he/she grows out of the trailercycle.
(My trailercycle is NOT for sale. I'm saving it for the next child)