The new Planet Bike Superflash Turbos are really bright. I've switched out the regular Superflashes on my commuter and road bike with these and have been told by quite a few people they were amazed how far away they were visible, even in bright sunlight. And I'd have flags on both rear corners of the trailer. Motorists aren't expecting it to be there so you need to make sure they see you are towing something.

I came upon and accident on my commute home a couple of years ago that occured right at sunset. The driver was facing into the sun waiting to make a left turn. Cyclist was traveling in opposite direction towing a trailer. The driver made the left just behind the bke and cut the turn close not seeing the trailer. When I saw the flattened trailer laying in the intersection, I was amazed that the child had survived.