Any decent bike will work for a trailer - they don't put a very large load on the frame itself as the majority of weight is carried by the trailer wheels. You might, however, want to think about a bike that would work well with a triple crank, as the trailer weight will REALLY show up on hills!
Other than that, I'd say get a bike you would love to ride without the trailer too. As you know, kids grow up really fast.
Oh, and make sure they frame is setup for full fenders - nothing worse than showering your trailer in grit and grime.
I have a LHT and like it for the long wheelbase and low bottom bracket. It's also great when I load 50 lbs of groceries on it (this is where the frame construction matters most) but I'd also look at the Cross Check from Surly, the Double Cross from Soma, or Jamis Coda. They'd all be a little more lively than the LHT if you're not hauling cargo.
Last edited by Rockfish; 09-11-12 at 09:01 AM.