When my ducklings were in the brooder I took a old juice container and cut a rectangle about neck height only big enough to allow them to get their head inside to drink. It was cheap and it worked like a charm.
EDIT: @dotknott beat me to it!
You can lower the heat lamp if you want. The ducklings will move away if its to warm. With that said, if your brooder is really 90 degrees I think you're fine. Like @dotknott said, there is really no need to get in the water at this point. I personally would not allow it, then you don't need to...
They won't need the mothers oil as long as they are kept warm with a lamp or some other heat source. I would be will to bet with the heating lamp in the brooder they should be fine.