Mine were pre-schoolers so can't help with a definitive list but...
Booster seats for your car
Board games/boxed games - these saved us on rainy days in those first few weeks of funneling. With a five yr old whose visited us Hungry hippoes is a always a hit and we have marble run (building an obstacle track for marbles) that is a hit with most ages and you can all get involved.
A kite. It doesn't have to be an expensive one, you get them for £1.50 in the Range! It's great because if you have a country park or something similar nearby you can go somewhere that's not crowded and overwhelming and pretty much isolate yourselves for funnelling, while doing an outdoor activity that isn't the park 
Paints, paintbrushes, painting aprons and a big sheet to put under your table and save newspaper now to go on top. IKEA if you have one near by do cheap paintbrushes and aprons. Tesco do poster paint at £2 a bottle.
If you have money and space and haven't got one, then a trampoline. That has been the most valuable thing we ever bought tbh.
We found we bought a few extra bits once we'd spent that first few days at foster carers and saw things they loved there. But you'll need to ask foster carer what's theirs first.
Oh....and as fellow adopter of siblings, plenty of things with caffeine in, for you obviously, not them!