The best way to introduce coins to young kids is to actually give them the change and tell them the value of each coin. But most often in 2nd or 3rd grade math these coins are introduced to the kids. If they had no previous knowledge of the change then it is very hard for them to grasp the concept.
So I urge to parents with young kids, please introduce your kids with all the coins. Take them to the dollar stores and give them change to buy small stuff there.
Once kids have knowledge of all the small coins then the following worksheets can be helpful for the theoretical understanding to solve small problems and conversion of coin value into decimals.
Visit the following page of my site to print more coin worksheets on USA and Canadian coins