How to teach her which finger to use?

Yes, Nadia is absolutely right.

Your DD right now is building eye/hand coordination and preoccupied with completely different problems (for instance, she has to match 2 different fields of view). At this point I recommend do not worry about what finger to use and joke that beginners (even adults!) can use even nose to play!
By learning piano key sequence and orders of notes with the Flash cards – a song and getting more familiar with the pictures and over all setting your DD finally would be more opened for fingers suggestions.
Playing the piano exercises also contributes big time to the ‘finger management’ situations.
Please, keep in mind that unlike all other existing approaches in teaching piano Soft Mozart letting beginners of any age to SEE a note and a key and their connection. Therefore, we do not start with the finger numbers since it is the way of playing keys and notes blindly relying on muscle memory.
All the students come to the right fingering naturally with no force.

NadiaD wrote: I see a point in bk question.
It would be very helpful if SM Team would make some videos in addition to Lesson Plan. Most of the parents here do not have any piano experience so this would be a great help!
For example :
Week 1-3: Playing HCB with R, L and both hands. Proper fingering and hand position.
The parents will have an idea how the playing of the song should eventually look and little by little try to teach the proper finger placement if child is interested, if the child is not interested, then just wait , and practice it by themselves!

Well, good point!
We are going to provide videos of how to play different songs!
Stay tuned!