Have you seen the Long Beach Board shorts??? They're a quick sew with awesome results. Great for any boy or girl for that matter. They're also part of the SewFab Bundle. Sooo many patterns, such a great price!
They have a flat front with elastic in the back, with an optional drawstring front. The piping detail in the back really helps the contrast in the fabrics pop. And don't be scared of piping, it's actually very easy to sew. Here's a great tutorial on piping: http://www.makeit-loveit.com/2012/01/sewing-tips-how-to-make-use-piping.html
These are made for regular woven cotton fabrics, but they easily could be made from knit or microfiber for swimwear.
And they're great on all sizes! Small to big kids!
Here's some closeups of the back. These would be awesome for any adventurous beginner!
And you could easily sew up a matching tshirt!
So go ahead and check out the Sew Fab Bundle! There's many other amazing patterns as well!