First of all, thoughts cannot be “stopped”, that is impossible. And trying to do so would be extremely frustrating and panic inducing I would expect. You can however think opposite thoughts. 
You might already know this, but for the sake of anyone reading that might not.
Second, we only wage war against things that are strong and powerful and need to be fought. Thoughts don’t need to be fought. Fighting them only reinforces the belief that they are powerful and must be defeated.
I would tell your daughter that it is okay to have “bad” thoughts. My understanding is that a major reason we are on Earth is precisely so that we may have thoughts that work against our desires without any negative consequences of an immediate nature. We are given a large buffer of time to pivot back toward something more constructive.
Furthermore, reality is not “neutral”. It is naturally biased toward the constructive. Good things will happen completely without our intervention, and being calm also naturally leads us into positive areas.
I am talking about external reality but let’s also talk about internal reality for a moment. There are layers to the self (not truly, but for sake of explanation). Hopefully I don’t muddle terms too much here, but the inner self has an “ideal” blueprint of your body’s condition. This is built-in. Your conscious mind is the boss, as we know from Seth’s teachings, what it says goes. If it believes that the body is faulty in an area, that message is received by the inner self, which does the actual work of reality creation, but it takes that image the conscious mind is sending it “there is a fault”, and it compares it to its own knowledge of the ideal condition “there is no fault”, and it notices the discrepancy. It then sends this knowledge back to the conscious mind, in intuitions, and dream symbols - to correct the beliefs involved so that they once again match the inner self’s own depiction of itself. Now you must believe that this information is available to you (it is) in order to receive it, if we don’t then we will block it out, but it is there nonetheless.
And even on top of this the body consciousness is geared toward health, and robust. Our beliefs will indeed manifest reality - otherwise we would never learn, but we have designed ourselves such that the body will often retain its health regardless through many means. So you see even the inner self has built-in safety mechanisms that are naturally inclined toward goodness.
Steps to change a belief:
- Know that we are not our beliefs, nor our thoughts, they are simply things we have.
- Know that our beliefs are not necessarily facts just because we believe them, they are not reality, they are only our ideas about reality. Stay aware of this, what we think simply “is” is actually a belief. Pay attention to the things that are thought of as simply “true”, these are beliefs that aren’t recognized as beliefs.
- Now that we’ve started to see these aspects of our experience not as facts but as beliefs, and we’ve begun to separate our identity from them, we can begin to call their authenticity into question, repeat “just because I believe this does not necessarily make it true”.
- There are a lot of methods here, you can begin to critically break the belief apart, reasons why it might not be true, and have it fall to pieces under your analysis, or, you can forcibly imagine what you desire, and feel the positive emotions flowing from that imagining, and use these and thought to substitute the belief with the better one. Or, you could make a mantra out of the new belief, repeating a particular suggestion.
There is no need for this to be difficult, many issues are simply unexamined, you need only poke at them a little bit and they will fall apart and the entire emotional and imaginative environment around the issue will be changed. When the belief is altered, positive, constructive thoughts will begin flowing automatically, and these will manifest the desired reality.