First of all, If she wants to, & you're ok with it, go for it. Now, in terms of personal preference, I vote no. I have 3 girls (7,5,2). We have never put them in costumes. The are itchy, cumbersome, & inconvienient to store, etc... What we do, though, is dress them in dresses. (Yes, I am old fashioned). They aren't frilly things, but very nice summer dresses or sun dresses, and put pretty matching ribbons in their hair. We usually have to go to boutiques to find dresses for all 3 to match (which they still like doing, so as long as they're ok with it, i'm gonna do it.) People always treat them so well. I truly believe it is because they are well mannered, and yes, well dressed. You do not see alot of little girls dressed up in the parks. But, they girls do not feel over-dressed. The dresses are very comfortable, and cooler than wearing shorts anyway.