Mobile Fence does not provide a service to block incoming/outgoing text messages.

However, you can block messages by using the 'Block phrases' feature provided in the default Messages app as follows.

Note that only 'incoming messages' can be blocked; 'outgoing messages' cannot be blocked.

After configuring the above settings, you must enable "Block Message Settings" in Mobile Fence to prevent your child from accessing the message settings and changing the blocked phrases set by the parent.