How can I teach the meaning of money and shopping?

Before calculation, teach the meaning of exchange—give money, get an item; things have prices—experientially through real shopping. Start with a small errand of just one low-cost item and practice paying with a set amount. Checking the price together to see "we can or can't buy this," and planning a wanted item with an allowance, ties it to daily life and makes it clearer. If coin calculation is hard, electronic money or handing over the exact bill are practical methods too.