WhatsApp works very well for what the OP wants. 

It's device and platform agnostic and can be set up on a device without a sim but does need a unique mobile number (a number not already in use on WhatsApp) for the set up. 

It's ideal for replacing SMS (text messaging) and MMS (photo messaging), very good for group messaging and provided you have good data connection/speed, WiFi or cellular, it's pretty good for video calling.

You can see when the message has been delivered and also when it has been viewed.

Once it has been set up on a device you can use the web version on a computer. Opera browser has a built in WhatsApp client. However I doubt in the OP's case the web version would be needed.