Girls, don't try to hem them with the elastic already in!
That would be a nightmare. You thread the elastic through after you hem them....lemme see if I can explain it, as it REALLY is easy....
Hem the end... but don't sew the hem completely closed. When you get back to the beginning, leave about 1/8 to 1/4 of an inch gap between the start of the hem and the end. This should make the hem stable, but leaves just a tiny open spot to access the "tube" the hem created.
Get elastic that is narrow enough to fit in the hem tube. Cut a piece off that will be longer than tyou think you need. attach one end of it to a safety pin. Run the safety pin through the gap you left in the hem up into the hem tube. Then thread the elastic through the hem tube, bringing it out through the hem gap again. You should now have both ends of the elastic sticking out the hem gap and the rest threaded through the entire hem tube.
Put the baby leg on the baby and pull the ends of the elastic tight until you have it the size/tightness you want it. Mark it. Take the baby leg off, cut the elastic to the mark, and then sew the two ends together to make the elastic into a circle (either on a machine or by hand... by hand is probably easier). After sewing it together, the sewed part should pop right through the hem gap into the tube.
You should now have the hem tube with elastic running through it!
Did that make sense???
I actually don't put elastic on the ones I make as my LO has uber chunky thighs. But normal baby thighs might need it. The only way to find out is to try.