My three year old daughter had a lot of fun - her hands are purple now!
I did this activity with my three year old daughter and it was dirtier, than I had thought. After painting the first painting, she began mashing the paper in the water and brewing a thick purple color brew she then spilt over the table. :D - But she had a great experience I suppose.
Btw. we didn't have tissue paper, so we used crepe paper, which also worked very well if we moisened the paper before applying it. (Laying it on the paper and watering it then, didn't work so well.)
Oh and I was so happy to use the crappy watercolor paper I cannot use for anything else!