I was asked recently by someone - not a noob, I might add - about where to find decent shapes and skins for male kid avs in Second Life. I was of course able to direct him to a couple of vendors which immediately sprung to mind. After helping him, he asked about clothes. And I kind of stammered for a bit. It's the same problem, believe it or not, that I ran into when I was asked a similar question right around SL5B by an adult friend of mine who wanted to try being a kid for a while, just for kicks.
The problem is, I can't really recommend any "boys" clothes vendors, for a couple of reasons. I'm not going to name any specific vendors, because I don't want a vendor to get all offended and come yell at me (although I suppose that by not being specific, I risk all boys' clothing vendors getting offended and coming to yell at me), but I have some problems with most of the boys clothing that is available out there.
Problem 1: For some reason, a surprisingly-small number of boys' clothes-makers have discovered the upper-most limit of the pants layer. I've encountered any number of pants, like the ones shown above, where the artist has almost meticulously failed to fill in the entire pants layer to the very top. Why is this a problem? Because, as illustrated, the bottom limit of the shirt layer exactly meets the top limit of the pants layer; thus, when your pants aren't "tall" enough, there's a really thin strip of tummy showing between your pants and your shirt. If the pants are really bad enough, your underwear shows. According to the unwritten rules of my childhood, your underwear showing in public is simply the most humiliating circumstance that can befall a boy of any age, and simply will not do. But even if you avoid the underwear problem by not wearing any (...that's right - so???), you still have that strip of skin there, which makes your shirt look too small (ironic, considering the shirt is as long as it can get and the problem actually lies with the pants) and you just look like a dork. To be absolutely fair, this is a problem with ALL male pants in Second Life, not just boys' clothes. The only workaround for an item like this is to wear a shirt on the jacket layer, so that it can extend down past the waistline of your pants and cover the awful glitch. But what if there's something else you want to wear on the jacket layer, like (imagine this) an actual jacket? With an open front, so the shirt beneath is visible?
Problem 2: There are some boys' clothes-makers whose jeans are so low, and/or whose shirts are so short, that it can't be written off as accidental and this too-small-clothes look is the intention of the artist. There is a significant number of these styles marketed to boy avatars; I can't really say anything about them save that even though I am boy avatar, these clothes are aimed at a consumer base I don't belong to.
Problem 3: There are some boys clothes that do manage to fit just fine, but they have cutesy designs on them which make it obvious that they're for much "younger" avatars than mine. In other cases, the clothes are much to "trendy" - i.e., they're ad-shirts for toys or TV shows or various products that appeal to kids, or they include baked-on accessories that don't appeal to me, etc. These things don't really interest me, so I avoid those clothes as well.
So what's left, then? Well, in the boys' clothing market - nothing, sadly. Therefore, I tend to buy my clothes from adult-av clothing vendors. And why not? Sizes don't matter in SL; the garment scales down just fine, and although (as mentioned) there's still the occasional brush with Problem 1, but it's easier to spot on the adult avatar models and thus avoid. All but a small number of the clothes in my inventory are "adult" clothes. And that's what I end up telling people who ask: until things change, it's probably better to Buy Adult.