Does toddler need a helmet inside a bicycle trailer?
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.