My daughter has white foam on her mouth when she is sleeping, what could be the cause and how can i treat it, also what can i use?

I would take a picture and review it with your pediatrician. Otherwise you don’t need to do anything if she is otherwise well.

https://www.sleepfoundation.org/sleep-faqs/drooling-in-your-sleep Actual foaming may be from medication, rarely rabies or seizures which likely would have other symptoms. Follow up with MD to make sure it is not something more significant.