The biggest thing to remember is to leave extra so you can let it down later. I hemmed his pants last year, and left at least 1/2" extra (tucked into the hem) to let down for this year. It's perfect! (At least for the ones that still fit in the waist!)
This is something our grandparents and their parents (etc. etc.) used to do-make things last, and I think as a whole, we've really gotten away from that. It obviously won't work if your kid shoots up like a weed, but with the buttonhole elastic in pants now, you can always buy a size up, tighten them as much as they go, hem the bottoms-and you should have pants that will last a couple of years.
It's very easy, and even a beginner sewer is capable of doing this. You just need to measure carefully, cut straight, iron using a seam gauge, and sew a straight line.