No. As long as you are the parent or legal guardian, and the child is under the age of 18 years old - you are legally allowed to monitor your child's activity without informing them.
Should you tell your child? That's up to you. Every family is different, and when it comes to protecting your children it's sometimes good to open a dialogue, and it's sometimes better to be discreet. Only you can decide whether or not a conversation is appropriate.