The problem is more that these things exist, but it's never what's popular. There are plenty of characters with big boobs in media that kids read. Shit, kids read manga and superhero comics, they've seen plenty. It's more that, in popular works, the characters tend to fall into the same archetypes. There's not a "unique" character who falls into the archetypes the child might associate with (whether that's a fat protagonist, or big boobs, or anorexia, or a race thing, or a religion thing, or some other thing), instead the character is either defined by this or fills a role that's expected of them. They're never the "normal" character, it's never someone the kid can actually relate to, except in media that's not popular and that the kids won't read in the first place.