I look at this as 2016’s prank calling with a twist. The game is called “Text Door Neighbor” and what kids do is take their phone number (for example 555-555-5555) and then send a text to the number that is one up and one down from them (so 555-555-5556 and 555-555-5557) and just start chatting.
Some times (probably most times) the conversations are benign, but this also opens up your kids to relationships that could be dangerous or unhealthy. The conversations could easily turn into meetups either online or in person and you, as the parent, would have no idea who the person on the end of this conversation is.
Always know what your kids are doing online and who they are talking to. To me, there is no issue with reviewing your kids texts or setting up a parental monitoring system through your provider. I have Verizon and I know they offer one that is only $5 a month.