I'm a casual Child trailer user - I don't think you can do much racing like Ben Hur once loaded with two sprogs. But its great!
I use a 2 seater "Chariot" made in Canada - I take my 5 year old on a hillyish route 7km to school alternate mornings. Its quicker than by car! 35 mins. Jo'burg traffic is Terrrrible
I occasionally take the two sprogs, 2yr @ 13kg and 5yr @17 kg on about a 10 km ride which works well. But granny gears are an essential !
Things to look out for easy QR / strong mounting bracket
weather covers wet and sunny
A trailer flag is good for visibility
A good parking break
Room somewear for the play room, reading room, dining room and kitchen sink and you are set!
Oh and don't forget to tune your brakes up - things get slower slowly with the extra weight!
We have had ours for 3 years and it has been excellent - it is quite wide but the fact you can pack a tog bag and a laptop PC and kids bag in the back is great for commuting!