She needs to brush with xylitol tooth paste, I don't know what you can get in Germany. If her teeth and very decayed you may not have any options as you don't want teeth infections and problem with incoming teeth.
There are dental pastes called MI PASTE, they do contain some dairy so your child cannot be allergic. They do help rebuild teeth.
I have strengthened my teeth with the paste.
If you apply then play a game with your child then spit out.