I am sure you have heard of Cloud Dough, if not you can make it with 1 cup of baby oil & 5 cups of any flour. Mix it all about & it will becomes a silky consistency, it will look a lot like soft sand but can be packed and shaped like smooth dough. You can add glitter or sequins to it if you want, we had some star sequins that got mixed in.
I poured the ingredients in to a tray and let Dimples mix it up & play, he told me it was the moon. So we went with that and made some moon craters, he had some little transformers & robot looking figurines that resembled the space men, aka Astronauts who were on the moon exploring.
He experimented with burying them, it worked well, under the fine and light cloud dough they gleamed through little gaps and flickered through holes in the "moon sand".
Then I got the idea of tunnels, we put the fire together and then put an ice cream scoop with the handle side next to the light. Dimples buried his lights, also burying the tip & handle of the ice cream scoop, I packed it down firmly over the handle & then slowly removed it out, leaving a circular tunnel leading up to the tea lights. It worked well, kneeling down and getting his eyes level with the entrance of the tunnel Dimples could see the lights flickering, then he examined the top of the mound- No light! He looked back down through the tunnel "Look mummy the fires are on in the tunnel".
The Flash on the camera mucked up the photo, it does it no justice, but you can imagine how it would look- no light on top & the flickering of tealights shining only down the tunnel. It was pretty cool! Dimples enjoyed playing with the lights under the dough and then experimented trying to make tunnels with his finger & with the handle of the ice cream scoop. Then he would squash them all out and start again.