Marking the planets

I'm helping some cubs get their astronomy badge tomorrow hopefully. So here we are testing out a way of measuring the relative distance of planets.
The sun is a tennis ball (far too big for the scale we're making, but more visible than the marble which is to scale!)
So far we have Mercury, Venus, Earth and Mars, Jupiter then Saturn by a spade in the distance. Uranus is on the way back as we had to wrap it around!