Does toddler need a helmet inside a bicycle trailer? I am thinking about buying a trailer for my 1yo daughter. They are quite common in my region (southern Germany) but I haven't seen a single kid with a helmet inside. I know that there is no law that regulates it. Does trailer's manual ask to wear a helmet? Or are there any official (not necessary German) recommendations about wearing it?
Generally helmets are recommended for carrier usage by manufacturers and governing bodies, although local cultural practices may vary depending on perceived risks. 

Helmets are recommended because it is still possible to crash or even roll a trailer, just as it is possible to crash on a bicycle, even if you are riding slowly and in quiet areas. While children should be wearing seat belts within the carrier/trailer, the structures would not provide adequate protection against head injuries, making helmets the last line of defense against a head injury in the case of a crash.