Wow I think a space theme would be so cool. I am coordinating some decorators to make cakes in the same theme for a school holiday program and a friend of mine is making a saturn cake next week.
When it comes to cakes I think Saturn is like the planet of planets lol. Are you making it 3d? That ring will make it a little interesting but there are a few ways it could be done. I can give a few ideas if you need.
For anyone looking for inspiration here is a pic of a cake Space Cake
This was made by a professional decorator but if you were into it you could do the planets in a similar way on a single tier or even get a custom edible image to use and it I think the kids would love it.