Solid color hoodie or sweatshirt, for him Black or Grey - For Her, Black, Red, Navy
Socks, - I would think more long term and get both ankle and mid calf style, so they can be worn with athletic shoes or boots. Black Friday you can pick these up almost anywhere and get more for your money.
Sleep Pants - I personally would look for plaid, patterns, or solids, as with teenager what they are into vary sooooo much, with that I would suggest a few long sleeve t-shirts to go along with them. I suggest Hanes, they last and are not really expensive.
Gloves for both.. for her you could go with mittens. maybe for her a scarf and mitten set.
Bathrobe - for both, if they are in foster care this helps with the modesty and warmth.
I know that a lot of kids are wanting those head phone things... My friends grandkids are all asking for them for Christmas. Lots of price ranges with these again Black Friday would be the time to pick these up.
Personal products are good, AXE for the guy, my nephew loves this stuff. Keep your eyes out for gift sets. Bath and body works for the girl, lots of options here.
Make up kit/ cosmetic bag - with nail polish, remover, file etc.
Shaving kit with essentials
You could make stockings for each - lots of little things that will add to their day. Maybe some school items, pens, pencils, colored pencils, markers. Personal items, deodorant, nice shower gel, lotion, razors, shaving cream. Tooth brush and toothpaste, nail clippers and file, add in some candy canes.