Just a plain earth mix might get pretty smelly and harbor bacteria, especially after being pooped in. If you want that sort of setup, I suggest researching bioactive substrate which includes an ecosystem that can break down waste. Otherwise, it's easier to have a solid substrate like shelf liner that you can easily clean and sanitize.
Their appetites fluctuate so it's hard to give a specific amount they should be eating. As long as they are eating some, are active/alert, and their weight is steady or increasing, they are probably doing just fine.