I decided it was fine, (after all, I had tried it on her a few times, and it fit, right?) wove in the ends, and then tried it on her again. Decided it could have been a little longer after all (like my husband said) before shaping started, so crocheted some edging on it. Tried ruffled edging for fun; it looked great, but didn't cover the ears anymore (which was the point of the hat after all). Re-did edging with plainer edging.
Bribed the kid with Smarties to wear it long enough for me to take photos. (This should have been sign, really. Since when is this kid reluctant to wear a hat? Perhaps when it's uncomfortably small?) It covered the ears better, but not perfectly. She played along for the photo shoot, and after a bit I realized that it had bunched up at the top a little, the reason being that the whole thing is just too darn small. I'll have to frog it entirely and start over, which wouldn't be quite so bad if I hadn't woven in the ends. Sigh
Anyway, I still love the pattern, which is blameless. After all, it's just that my kids have big heads (in this case, just barely short of 19"), and I didn't want the gauge any looser than the 6.5 sts/inch that I ended up with (using sport weight yarn and 4 mm needles). I'm thinking I should really just make grown-up size hats for the kids from now on, so perhaps when I start over, I'll try with two pieces of this yarn held together, bigger needles, and hope that I don't run out of yarn.