how can i check my child text sms

Unless your carrier offers some soft of services that allows you to do that, the only thing you can do is physically take the phone from your child and read the messages.

There was a service that allowed you to spy on someone's cell phone, however, the phone had to be jailbroken, which puts the user (or child) at much greater risk than anything that might be contained in a text message. And the company's database was recently hacked, and all personal information from 4 million phones is now for sale on the Dark Net. And the stolen information is now being used to blackmail the victims (see Brian Krebs' blog today: http://krebsonsecurity.com/2015/05/recent-breaches-a-boon-to-extortionists/)