It’s great that you are supporting your daughter and that she is doing so well.
Oxbridge admissions is not really a mystery and there aren’t secrets. The information on their websites is all you need to know. Nearer the time, you and she can go to open days and ask questions there too. What they are looking for is bright, highly motivate, and teachable students with the skills and aptitude needed for their chosen course. There aren’t special criteria or things you have to do beyond that, and definitely not anything that needs planning from age 11.
Also: try not to fixate on two universities, especially this far ahead. She may not even want to go there (or she may want to do a subject that isn’t offered there). Even if she does, setting it up as a goal so many years ahead could make it harder if she doesn’t end up getting in.