Yes! We are going to hem before we sew. The biggest reason that I recommend hemming first when you sew knits is that it's going to give you a much nicer looking hem. Sewing something flat is always easier than sewing something that is in a loop. In the case of knits, you tend to get the most stretching when you sew over a seam. Since we're eliminating those side seams, you won't have that problem! Don't worry at all about it looking odd. I promise that if you go dig through your closet or child's dresser, you'll find that almost all ready-to-wear knits are sewn this way! And, I'd bet that you never noticed it!
Personally, I like hemming first because it's my least favorite part of sewing knits. When I first started sewing knits, I tried to finagle ways to use cuffs on everything! (Next week when we do the Charlie Hoodie, there will be no hemming!) This way, though, the part that I like the least is done first and out of the way. (It's not really that bad, though. I promise!)
I'm going to show you how to hem using a twin needle here. If you're not quite ready to try out the twin needle, head over to my blog, where you'll find instructions for using the triple stretch stitch to hem your knits: Simple Knit Applique & Single Needle Hemming Tips. (There's also a great tutorial for knit appliques in that same post!)