Here's what I did: I took a paper plate and squirted out three colors--red, yellow, and blue (I also added white so that we could see how combining white with any other color makes a paler shade of the same color). Then I gave each of my children a piece of finger painting paper. I worked with my three year old to add circles of single colors and then add another color over top and mix to see what happened. He was absolutely delighted when red and blue made purple and yellow and red made orange, etc. We discovered it was just as easy to simply dip his finger in two colors and then mix them on the paper at the same time. He just loved this activity, and I loved that I could see the wheels in his head turning as he experimented with combining two colors to create a new one.
My two year old had a great time with the finger paints too. What I like about finger painting is that it's such a tactile experience for the kids. My two year old especially likes squeezing the paint between her fingers and rubbing it in globs on the paper. It doesn't matter if it doesn't look all that great in the end. This activity is all about the journey, and not the product. I hope your little ones enjoy this as much as mine did!