Was the planet destroyed by the cherub's dad the new planet the the kids created? (I thought that was said earlier.) And if it was, how was it populated by trolls? The only relationships that existed were trollxhuman not trollxtroll.

No it was just a planet in the C universe that trolls would eventually inhabit. And trolls don’t make baby trolls in the same intimate manner humans do. They just have to get all their baby fluids and mix em together and the Mother Grub will do the rest.