The easiest way is to look on their phones. Since the phones are technically your property (you own the account, after all) you can take them for this purpose.
The next best way is to ask the carrier for a log. This will not tell you the content of the messages, but it will give you a list of phone numbers that their phones have text messaged with. If you obtain a court order (which you can in certain situations depending on your location) you can receive the actual text of the messages they have sent and received. This is not guaranteed to work, however.