It's a little pricier than what you'll find at Target, but my little guy (3, and small for his age) wears one of these:
http://www.specialized.com/us/en/ftr/helmets/youth-helmets/small-fry-toddlerHe loves it, and I think the flat back makes it more comfortable to wear in the trailer. The back knob is easily adjusted by little hands, which is good because he can "help" put it on even though he can't really do the buckle, but also bad because he can loosen it while wearing it.
We insist on the helmet in the trailer. While the risk of head injury is low, so is the cost (extra seconds of time and minor inconvenience) and it helps reinforce that "while biking, helmets shall be worn". Also, while I love my InStep trailer, I'm not sure I totally trust the frame to act as a roll-cage in an accident.
As for getting the kid to wear it, if it's important to you, this just needs to be a non-negotiable. Presumably, if the kid is old enough to remove the helmet he/she is old enough to understand why this is important, and that this is one of those rule that shall not be bent. Make it clear that if the helmet comes off, we go home now. And then do it. As with anything else, you need a clear, reasonable expectation, logical consequences, and follow-through.