How do you make hunger go down in minecraft but without monsters spawning?

If you have cheats enabled, you can do the following:

1. Make sure the difficulty is set to Easy or higher.
2. Type in the command: /gamerule doMobSpawning false. (This is case sensitive!)

Now monsters will no longer spawn, but you will still get hungry. This works because, when you set doMobSpawning to false it prevents monsters from ever spawning, and when you set the difficulty to anything other than Peaceful you get hungry.

Monsters will still spawn using Spawn Eggs or from Mob Spawners (including those naturally generated in dungeons), but they will not naturally spawn.

Source: Minecraft Wiki