I like the trowing a ball against a speeding train analogy.
The important factor is that Jupiter is moving, quite fast. About 13 km/s. The other important factor is that Jupiter is very massive, so it has a lot of gravity and it can draw objects towards it, causing their direction to bend.
The tricky concept for a 6 year old is that gravity is a zero-sum game. The object that falls towards Jupiter, in this case it's a hyperbolic orbit but regular orbits work too, so as it falls towards it, it adds velocity then flying away from it, it loses the same amount of velocity, so the net velocity relative to Jupiter is zero. No change.
But because Jupiter is moving, Jupiter can tug the spacecraft along with it. The velocity relative to Jupiter is unchanged but the velocity relative to the sun changes, Jupiter is like the train that the ball bounces off of.
The opposite can happen too. Jupiter can be used to slow a spacecraft down. It depends on the angle of approach, like throwing a ball against a train while it moves away from you.
You can also do this experiment below (with 2 balls not 3 to represent the gravity assist, though it's a cooler experiment with 3). It's the same principal. Bouncing off a moving object (Jupiter) is very different than bounding off a flat surface that isn't moving.
All that said, I'd still point out that this is a tricky concept and not to worry if they don't understand it.