I am going on holiday to Mexico - What is chargeable when abroad when using my I touch?

Facetime and iMessage are free from any charges in and of themselves regardless of where you use them. You will of course have to pay any appliable charges for the WiFi and Internet connection.

Regarsd.

SMS text messages will always incur a charge, as they require and use a voice channel. So you would incur some roaming charges as you would have to connect via a local carriers network.

If you stick to wifi, and only free wifi hotspots, you can avoid all charges for data. Just turn on airplane mode, then re-enable just wifi. That leave the cellular radio powered off, and your only possible means of connection is wifi. You cannot send SMS texts then, but you can send iMessage texts between others with iOS devices and iMessage accounts. And you can use email without any charges then too.

So, as varjak paw says, it is not the app per se, but the data connection they happen to be using. If you allow them to use cellular data, then there will be data roaming charges. If you restrict them to just wifi, and only use free wifi hotspots, then there will be no charges. But, SMS text messaging specifically, always requires a cellular voice connection, as it uses the voice channel, not the data channel.

The iPod touch doesn't send SMS messages, so unless the OP has a third-party app, that wouldn't apply in this case.

Regards.

You need only a WIFI network to send the iMessages or use FaceTime.