Hello M.,
Since this is an old request, I don't know if you are actually looking for additional posts, but I wanted to throw something in here that wasn't previously mentioned.
I think it's great that you want to start educating your daughter about her period before it comes, and the discussion about her having the options both pads and tampons is necessary. However, since she is so young, I would be concerned about her using tampons, as they come with a lot of responsibility that she may not be ready for at such a young age. Tampons need to be changed quite regularly, and if her period is not heavy and there is no leakage, it is easy to forget that you are wearing one. Wearing a tampon for longer than 8 hours is not recommended as it can causes toxic shock syndrome - there is an insert in every box that can help you explain when it is necessary.
I don't think that a tampon can hurt her physically in anyway that is lasting, but she needs to demonstrate that she is responsible for herself, which is something that sometimes comes with time and can't be given an age designation.
Anyway, just wanted to throw this in. Best wishes - your daughter is becoming a woman!
(Oh, and just wanted to add that it is incredibly refreshing that you have such a great relationship with your ex and his wife - my parents divorced when I was very young and were never really able to maintain any sort of relationship that was civil. I think having an open relationship with them is giving your daughter a HUGE bonus!)