I have T-Mobile. You will require the phone to be active (not airplane) in order to use the text over the network. Unless you manually turn off data, your phone will look for service constantly and it will rack up your bill. Also will drain your battery.
Royal’s app allows you to text for 1.99 per person per day. I’ve never used that but read others with some success.
Maybe I’m a sucker but I just buy the 4 device plan for our family and consider it a cost of the vacation. The texting over WiFi gives me more confidence for my teens. Everyone has their own needs and comfort level, so just educate yourself and make the best decision for you.