She'll get a decent view wherever she stands, plenty opportunity to walk about and decide for herself - there are "big screens" at some points, so she could watch them approaching and the end of the race. When I went I walked about, stood for a bit and spoke to people and then moved on - I think I had a different view for every lap. Tell her to not get too hung up about staying in one spot!
When I went it was Lance-era so lots of Yanks who were a good laugh and were very good with my elder daughter who they ensured got to the front of the barriers to get a great view, they were not too happy* when I shouted to my daughter "that's him, in the yellow jersey, the doper I was telling you about..."
* actually they did have a chuckle, when they turned round to find who had shouted it and found a big idiot Scotsman half way up a lamp-post grinning like a loon