If I understand the terminology, The Peace of Mind Connect Plans allow the connection of other SIM-enabled DATA CONSUMING devices to be added to the plan. If the tablet uses a data-based texting application you'll be fine. If it uses traditional SMS, then I would expect it is not included.
Note that the Peace of Mind Connect plan provides mobile data only for your other devices. It does not provide a phone number which is required for SMS texting service. Only the phone associated with the plan can use the unlimited calling, unlimited text, voicemail, etc. because only your phone has a phone number.
Under each of my connected devices there is a phone number associated which each device. Also under plan description of each connected device it says text messaging services included. Which made me think it must be sms also?
I had the simple share plan plan added to my tablet and didn’t realize texting was costing extra but under this plan as in the picture it says text messaging services so I was trying to verify if I text with this plan on the device will I get charged extra. I’m probably going to have to bit the bullet and call in to verify lol