Usually, use this to help learn borrowing and carrying, but if you're having a hard time figuring out coin value, this might be helpful. (particularly, if borrowing is already a mastered skill)
So, here I am trying to subtract 16 from 32 (32 - 16).
You can't do (2 - 6), so you borrow a dime and make it into 10 pennies.
Do the subtraction and here's what's left.
~ Once you've got these 2 coins down, add a dollar or one more coin at a time.
~ Each morning, have a pile of coins that can be counted up. Use only the ones that have been taught, but keep adding more as you can.
~ Go to the grocery together, have them buy a small item with only coins and require them to make exact change. (go at a slow time)
~ Use real coins instead of play coins, you only need to have a couple dollars worth on hand.