My son and I used to do this all the time. Rubber duckies are great for young toddlers to make prints with because they are large enough for fumbly hands and make a squeak when they are pressed down, so it encourages the child to keep doing it. What I didn't know was that at 3.5 he'd still love it.
1. Gather your materials. You will need a rubber ducky, some paint, a plate and paper.