Loads of good advice on here. Agree with all the stuff about keeping food etc the same. I'd use same fabric softener if the foster care does (if they say we just get whatever is on special then of course do not worry - although we have made a little ritual of sniffing fabric softener and washing up liquid at the shops, me and the kids, and they choose what we buy!)
Agree with Maryz do not overload them. It is easier to add bit by bit or rotate toys (into garage etc than have all out all the time.
In your shoes (sorry if someone has already said this) I would but a large plastic box (about the size of an old fashioned TV) and fill it with safe clean junk for junk modelling. EG cereal and tea boxes, small plastic bottles, larger plastic milk cantons, plastic pots, coloured paper, chocolate boxes - the whole thing the outer and the inner tray, etc. AVOID anything sharp, not just glass or tin (of course) but also those yoghurt pots that have sharp edges! Avoid anything that has had something smelly in it, e.g. ready meals or frozen fish where the meal was touching the cardboard. Go for variety of shape, size and colour. In a separate place (e.g. a drawer) keep some child safe scissors, glue and Sellotape - the kind you do not need to cut (e.g. Proper Sellotape), the kind you can break with your hands. It is more expensive but heaps easier for kids to use than cheap ones that do not tear. You could also use masking take which is easier to use.
To give you some ideas my kids, 4 and 10, have made a piggy bank, a model of horse, a dog, a game (which just needed dice to play - e.g. a board game), a robot I think, and paper wing (I think the other one is coming so no flying yet!)
If you get offers of money for a gift and have a garden, I can totally recommend a big item they can use together, a trampoline with safety net IS expensive but we have had hours of fun and it is very good for stress busting. They can even use it in raincoats in the rain if they really need to get some energy out.
We also got a fabulous paddling pool and a kind of runway that sprinkles water and you skid on it! The gifts that they can share can be very good for promoting teamwork and playing together, not always easy, but useful.
Good luck and congratulations.
PS My son is nearly five and needs a step stool, get one first and if you have a downstairs loo and another is needed get one for up and one for down as it can be frustrating if you have a downstairs loo they struggle to use.