I don't think its too childish at all, I think it's a very charming piece of clothing. I like polka dot stuff haha, some might say its too childish but they're closed minded. I just think it'd work and I can't really put my finger on why.
i think it is the shape of the top that turns me off to the whole swimsuit. reminds me of those straight across tops that 5 year olds wear when they wear 2 piece swimsuits. though I could see it working if you have larger boobs, like c or d cups because then you wouldn't show too much cleavage.
but really, wear it if you really like it. my friends give me a hard time for wearing things they consider childish like this watch. I have a nixon timeteller and it is bright blue, yellow and hot pink. but I love it and wear it anyways. rock your style, it is great to be unique. I'm sure almost no one would have the same suit as you because many girls opt for as little material as possible