The Boy and The Girl are hitting new milestones.

Last week we took them to a local park, where Santa held court near a restored old carousel. The Girl was in her glory – she told Santa what she wanted, declared which horse she wanted to ride, and pretended that her horse was jumping hurdles as the carousel turned. She smiled and laughed the entire time, and all was right in her world.

The Boy was okay with Santa, but begged off the merry-go-round. I sat with him – I lost the ability to do circular rides without bad side effects somewhere around thirty – and asked him why. He told me, a little wistfully and a little nervously, that he was too old for merry-go-rounds now. They're for little kids, not him.

The parents out there know that feeling – the peculiar mix of pain and pride you get when your kid leaves a stage behind. There's a loss in knowing that the little kid you've spent the last few years with is gone, but it's gratifying to see your kid grow and mature, and start to fill out the outlines of his personality. It's both surprising and not. TB is a thoughtful kid, in both senses of 'thoughtful.' He's polite and genuinely considerate, but he's also a thinker. (Between his parents, he comes by it honestly.) He's observant enough to notice that none of the other kids on the merry-go-round were his age, and status-anxious enough not to want to be the first. As a considerate kid, he didn't want to ruin the experience for TG, who was obviously enjoying herself. But he's just not temperamentally capable of faking it, either. So he sat out.

I remember that. Yup. I remember that real well.

He's old enough now to start worrying about things that never occurred to him before. Some of the confidence that comes from obliviousness will start to elude him. He's heading toward those awful years in which the rules seem scarily fluid and murky and arbitrary, and nothing feels quite right. And we can't shield him from that, as much as part of me wants to.

The contrast with TG is striking. She's amazingly secure in her world, and just happy to be there. She's still young enough that the rules are clear, even if some of them are clear only to her.

An exchange she had with Santa:

Santa: What's your name?

TG: [TG].

Santa: And what would you like for Christmas, TG?

TG: I would like an Easy Bake oven and a cupcake maker, please.

Santa: What does a cupcake maker do?

(pause)

TG (deadpan): It makes cupcakes.

She didn't actually say " duh," but the implication was clear. Everybody knows what cupcake makers do!

Her preschool had a Christmas concert, which was utterly charming. Hearing ten four-year olds sing "Jingle Bell Wock" is good for the soul. She wasn't the least bit self-conscious about standing in front of a roomful of adults with video cameras, singing and dancing and banging sticks together in approximate rhythm. She knew – correctly – that she owned the room, and that snacks and hugs and smiles and approval were in the offing. It wouldn't occur to her that it could be otherwise, any more than it would occur to her that someone would have to ask what a cupcake maker does.

That won't last. As she gets older, she'll hit the same anxieties that TB is starting to hit now. The rules will get more complicated and less fair. People will disappoint her, and she'll discover self-doubt. She'll discover someday that some problems are beyond the power of parental hugs to fix.

But not yet. As proud as I am of TB, I can't help but feel a loss as he moves from the untroubled years toward something else. He won't be the heedlessly brave little guy anymore. I miss the heedlessly brave little guy sometimes. TG is in the full flower of secure, oblivious courage now, and TB reminded me of how quickly that passes. I owe him one – yet another one – for reminding me to enjoy TG's cloudless certainty while it's still here.

Over the months and years, we'll watch TB grapple with a world that gets palpably bigger seemingly each week. And we'll take real pride, as we already do, in watching him become himself, and watching him navigate his world's sharp edges with his particular grace.

But for now, for Christmas, I'll sit through a few more versions of Jingle Bell Wock.