Lifting your daughter over your head and listening to the sound of her laughter, then ignoring the fact that your arms are tired when your son asks for a turn, because it's fair, and anyway it's worth it to hear him laugh, too.
Sending your son to "time out" because he threw a ball at your face, even though you had just told him not to; listening to him scream about not wanting to, but not giving in; ending the whole thing with an explanation and a hug; and knowing that, even though he'll do it again, he means it when he says he won't.
Getting choked up at the card your son "wrote" for you, even though in it he says that you're 12 years older than you are and makes a big deal about how much you like to eat beans.
Taking a moment to marvel at how tiny and adorable your daughter's feet are--even though they are so much bigger than they were just a few months ago--then having your reverie interrupted by noticing that her toenails are getting long and wondering whether to go get the clippers or just bite them.
Ultimately, a difficult thing to summarize or quantify, and maybe the most important thing you've ever done.