You can certainly do this, but you might want to think hard before you start. Obviously, how you decide to deal with your own children is none of my business but speaking personally I am very much against parents spying on their children's private lives.
All children who have phones send texts all the time, it's something you can do nothing about.
Reading your childrens' text messages is not going to stop them from falling into bad company, it will simply damage your relationship with them. It will send them a big signal that - despite what you say - you don't trust them. By signing up to a monitoring service you are spying on your children without their knowledge - unless you tell them, they won't know, and if you do tell them you are defeating your own purpose - they'll make sure that they never send or receive anything incriminating. They'll find other ways to get around the problem if they really want to.