I have the Walk the Plank ones because they were free. P4P just released their cozy pants pattern, for women and kids. Those look comfy! But already very similar to a pattern I already own (Portlander pants from New Horizons on Etsy), which is also very comfortable and are actually the pants I'm wearing right now, haha. But I don't think they have a kids' version.
Oh and Melly Sews has a whole bunch of free patterns (you just have to subscribe to her newsletter for the password to her pattern gallery) that includes pajamas! I'll start you off at this link but I know they're in there somewhere. mellysews.com/tutorials/