This is a great simple introductory print-making project for junior years, however one you could try with any age kids and have great success. The idea came from Whip Up and you can check out the tutorial there.
I had my grade two students wrap one long piece of wool (or yarn or string) around a wooden block, then tape it down to create a simple stamp for creating prints. They then used a foam roller to roll paint in a colour of their choice onto one side of their block, after choosing a piece of coloured paper they then created a patterned piece of paper full of prints. I allowed them to be as creative as they wished, so some students ended up using two or more sides of their block so they could create multi-coloured prints. Some even experimented with using the unwrapped end of the block to create prints.