I don't believe that children have to be anything other than children when they are very young. And my children are quite young (9 months and almost three years) so they don't have much of an opinion about their style or manners. But I do have some observations.
My two year old son is most elegant when he doesn't know you are watching. If you catch him relaxed, lounging, and completely at ease, he is so natural and at one with the universe. That has to be a definition of elegance. This also happens when he is engrossed in play. When he is engaged in anything simple and elemental, like running through sprinklers, he is also elegant (although cleaning up afterward may not be). He can get away with wearing anything at all (today, it was orange shirt, red boots, and green and blue plaid shorts) or even nothing and look cute. But when he is dressed up, he is quite a dapper little man. He is also elegant when his manners slip out: "How was your day, mommy?" or "thank you." So there are plenty of times that we endure spilled milk and toys all over his room, but he can be quite elegant in spite of that.
My daughter, the baby, is still a little baby. I don't think that babies should wear too many frills or be too punk rock (i.e. no black). But that quiet alert stage of newborns, when they are calm and just look around, is magical. And the baby's giggles with her brother's laughter at the same time is also, perhaps, the purest sound in the world.