I don't like to be a hater, but in my opinion the Weehoo trailers are "BSO's". I was considering buying one, and emailed their customer service with some basic questions about the gearing to see what speeds my kid would actually be able to "push" at. I asked for chainring size, cog size, wheel size, and dropout type.
They told me they had no idea what the chainring and cog sizes were, but that they could "ask the factory". They also said that the dropouts were vertical. I eventually ended up borrowing one from a friend, and found out that the dropouts were horizontal (as you'd assume for a singlespeed). I also found out that it was super heavy, it's basically made out of the same tubes you'd use for a trampoline frame. The Weehoo weighed significantly more than my bike did.
Sad, really, as I'd assume there would be a market for "recumbent" style bike trailers that don't weigh twice what your bike does. I ended up buying a FollowMe tandem. That's also heavy, but at least has some good options for growing children.