I'm trying to figure out a nice way to describe to a kid the physics behind these experiments:
Assuming ideal conditions, we have a small boat with a sale, close to a lake's shore and a fan fixed on the shore, oriented towards the boat's sale. Once the fan is turned on, the boat will start to move slowly away from the lake's shore.
Now, let's replace the fan with a small kid having a long wooden rod in his hand and let's set the boat back in the initial position. Once the kid touches the boat's sail with the rod, the boat will start moving in the opposite direction, slowly...
In the next experiment, the kid notices that his rod is not long enough to propel the boat further away, so he decides to climb in the boat and to try the same thing he did while he was on the shore: push the sail gently with the wooden rod, but, surprise: the boat did not budge an inch.
The kid thought that maybe the rigid nature of the wooden rod is to blame for the failure of the experiment, so he decided to try again, this time by placing the fan on the boat, oriented towards the sail. Did the boat start to move? Why?
Now, I could try to apply linear momentum theory and consider the air as being formed by small beads, each having an impulse $ p_i = m_i v_i $, where the masses and the speeds of the particles are roughly equal, and each $ p_i $ gets transferred to the sail, but I'm not sure if this gives any reasonable intuition to an elementary school kid.