Nope. We did for a while. The back of the seat pushed the helmet forward: that not only made the kids far more uncomfortable than a helmet normally would, but it also made the helmet slide forward in a way that impeded it's proper function making the kids' misery fruitless anyway. They are fully suspended in a harness inside the trailer - if something did happen to turn the trailer, they would not hit the ground anyway. The only way they'd get hurt in there is if a car crushes the whole trailer... and in that case, the helmet won't do a darned thing for them.
Despite not wearing one in the trailer, my kids always wear a helmet riding their bikes anywhere except in our yard or on playgrounds. They not only do not argue with me about it, they gave *me* grief when I didn't wear mine to bike to the neighbor's house a block away one day. Likewise, they are properly buckled into their carseats anytime the car is in motion, except when we're on site at an SCA event when they get the luxury of sitting in the front seat (maneuvering at 3mph through crowds of pedestrians and tents), or on the occasion that I have to move my car within our long driveway without exiting onto the road (say, if dh needs to maneuver something large into his shop so I need to get my car out of the way). Again, they have never ever attempted to unbuckle themselves or give me grief about it just because there are those exceptions to the rule. I give my kids credit for being able to understand complex rules, and they haven't failed me yet. I don't underestimate their intelligence by thinking they won't understand they have to wear a helmet on their bikes just because they don't wear it in the trailer. Honestly, I don't see why that complex rule would be considered so complex anyway - they're not on a bike at the time, I am. But, kids are people, not dogs. They understand complexities. You don't have to be utterly absolute all the time lest they fail to understand a rule's importance.