Halloween<br>Paula had decided to take her two year old trick or treating. She was excited to pick out a costume. Halloween night she dressed up her child and headed out. They walked all over the neighborhood getting candy. They both had a great time.
Paula wore a costume
The context only states that the child dressed up in a costume, not Paula. She might have done, but we don't know from the context. The system likely assumed both the mum and child dressed up at the same time. 
( z1 wear :ARG0 ( z2 person :name ( z3 name :op "Paula" ) ) :ARG1 ( z4 costume ) )