Can good aligned players fight good aligned monsters?

I recently introduced my nephews to D&D and they are level 9 (Ranger and Druid). The usually adventure with a level 9 Paladin NPC.

They really, really want to fight a gold dragon after finishing their last module but as a very rusty DM I am not sure how that would work since they are aligned the same. And what would be a reasonable challenge for a group like this?