Imperfect is right. Do mention it at the office.
The child (A)in question will know who the other child (B) is and, even if he doesn't get his hat back, (or if it wasn't his hat) child A's parents will know that school is aware of the problem and is ready to do what they can. There's chance for 2 lessons in school, one in manners and the other in honesty, both of which come under the heading of PSCHE. The school, when they send out the next parental questionnaire should also be prepared for lots of 'disagrees' and nasty comments from the parent whose behaviour is being questioned.On anonymous questionnaires requesting parentfeedback - paper or on line, some parents post the most appalling accusations or make the most ridiculous statements, which, if they are true, the parents should have been into school and had dealt with.
I do have 2 other queries. 1. The boy is referred to as a 'little boy' - where were his parents? 2.Although I agree with the concern about bringing sentimentally precious things into school, should we teach children that what we have or what we wear is determined by what others might do?