Congratulations!! You are superstars 
I would say as a general rule - get at least two of everything practical that you use daily.
My suggestions are artsy:
Lots of books.
Colouring books.
Lots and lots of drawing paper, craft paper.
Buy big packs of printer paper.
Paints - watercolours are a nice sensory experience, but also little pots of poster paint. Selection of brushes in different sizes.
Finger paints (check for washability).
Crayons - buy lots.
Colouring pencils and HB pencils.
Chalk and blackboards, also outdoor chalk.
Easels.
Whiteboards and markers.
Oilcloth for tabletops.
Smocks.
Clay and clay tools - playdoh, also plasticine and clay that can be baked for permanent artwork. Rolling pins and cutting and squeezing tools often come in Playdoh packs.
Glue.
Safety scissors.
Tape.
Mod Podge.
Coloured tissue paper.
Straws.
If you can get a handprint kit, a family handprint creation might be lovely for you all.
Scrapbooks for each of them so they can compile 'my family' narratives. Put photos from their early years in their scrapbooks (if you have them) as well as photos from now on. Little mementos can be used instead of photos.
*Take stock of what you have before embarking on this. The lack of documentation from early years items can be a source of pain for children.
Frames to put their art in. You can get frames in Ikea.
Magnets so you can hang art on the fridge.
If you don't already have them, a hand mixer, mixing bowls, a collection of cake and cookie recipes plus tons of sprinkles.
Little aprons.
Baking pans.
Pans for messy activities like baking broken crayons.
Bubble wrap.
Pom poms.
Craft sticks.
Sensory stuff in general - things that are interesting or beautiful to look at or touch like big sea shells, sand for the garden, little digging tools (trowels, etc), maybe pots to grow seeds in.
Little watering cans.
Family T-shirts and tie dye kit.
Some good 'process' art ideas here.
www.themeasuredmom.com/a-z-process-art-ideas-for-preschoolers/
I would also suggest a personalised chest or box for each of them to keep treasures in.
Little household tool sets - child size brooms, dustpan, dusters. They can help you do chores.
Big kitchen noticeboard to pin family schedule, shopping lists, and little pictures they create.
Safety - make sure chests of drawers, bookshelves, and TV are all attached to the wall.
Put a bolt up high on your front and back door, and make sure your windows have safety catches.
The children might decide to go visit the foster family or other people they know from their earlier years. Make sure they can't get out without you.
childcrisisaz.org/5-incredible-benefits-of-art-for-kids/