how do i monitor my child's text messages on her iphone without her knowing

That’s because you are sharing an Apple ID between different people. Apple IDs are personal; they shouldn’t be shared any more than you would share a toothbrush. If you share an Apple ID with someone else many bad things will happen. Each user should have their own Apple ID, then you can create Family Sharing→What is Family Sharing? - Apple Support. That way you can share what you want to share and not share what you don’t want to share.

It is OK to share an Apple ID among devices used by one person, such as an iPhone, an iPad and a Mac; in fact, it is a very useful thing to do. But not between devices used by different people.

Here is Apple’s FAQ: https://appleid.apple.com/faq/#!&page=faq. See the 4th bullet item.

For your specific issue if you have one device that shares an Apple ID with your child’s you can enable iCloud Messages by going to Settings/[the account name]/iCloud and turning on Messages on both your child’s phone and the device you are sharing an Apple ID with. You will have to watch it frequently, because any messages the child deletes will also be deleted from the shared device.

There are some, but they all require jailbreaking the phone, which is a much greater risk to your child’s privacy than anything they might post or receive in a text, because it would allow anyone with minimal skill to spy on the child’s phone.

Why do you want to keep it a secret? Why don’t you just ask your child to see what is on their phone?

You can’t.