As a childcare worker, I would say no soaps, candles, bathy stuff, choccies....we get sooo much of it at this time of year!
Things that I like to get are nice hand cream (our hands are always in water!), story books, other resources to use at work (like puppets etc...), heartfelt sentiments written in a nice card, homemade bickies/goodies.....
As for who to buy for, a centre gift is nice, though if you truely feel that his main carers have made the difference for your DS, then just get them something and give the director a card for the centre, and thank the other carers in that.