I should have taken some pictures along the way, but I didn't so hopefully these pictures help...it's very simple to do. The idea is just to add length to the body, arms and legs.
In our house kids tend to grow taller faster than anything else. So the jammies still fit in the waist but they get too short in the arms and legs and tummies catch a cool breeze.
Soooo, onto some instructions
-find some similar matching (or not) fabric. Try to use a similar weight of fabric as the original jammies are. (these pj's are flannel and I used microfleece, it worked fine)
-cut into the pj's. Just a single straight cut across each arm and leg and across the torso of the shirt. You're not removing any fabric here, just cutting straight across to allow you to insert the new fabric.
-using the pj's for sizing cut the fabric inserts, add about 1/4" to allow for seaming along the sides too. Make the fabric as long as you need, I'd say I cut mine in the range of 3" and that's giving my son plenty more room to grow.
-sew along any side seams on your fabric inserts
-next pin your new fabric into the pj's and sew them in (see the top photo)
That's it! It would've been easy enough to go buy some new pj's (but honestly I'd have rather not!). These pajamas were thrifted and my son has been wearing them for about two years, now they'll last another stretch and likely be handed on to a nephew.