First, in the unrated sections, almost all kids are beginners. They are not taking the games very seriously. After the tournament, there is no trace left to tell if they have played poorly.
Second, because no one has a rating, even if your child wins a section, you don't know how good your child is.
Third, at most tournaments, the entry fee for unrated and rated sections are same, so why not get a rating?
See my other articles about the benefits of rated tournaments.