Talk to your kid, ask her what she wants to get out of the experience, what she expects it to be like.
Then I would offer her:
Watching one of the big screens in the parks, which will mean a very early start, crowded tubes, possibly sitting on damp ground in the drizzle, limited access to toilets and a VERY long day.
She would however have the experience of going and being 'part of it all', and would see a decent amount on the screens.
Or:
Stay at home and have a Coronation Themed day, watching it on tv with family/friends, dress up, themes, buffet etc... still a part of it but at a distance and no early start, no crowds, access to own loo etc.
If she thinks she wants to be part of the parade route (do not offer this, just see if she raises it) then point out it will be the same very early start, the crowds will be worse and there is zero guarantee of actually getting somewhere on the parade route so may well be re-routed to the parks anyway, and if you did... still no guarantee she would see anything at all (being I assume, child height) - if she DID see, it would be 30 seconds or so of the King passing amidst a lot of horses, sandwiched between a lot of absolutely bugger all, and lots of police cars.
I wouldn't but, I am not the muppet who said 'Ill consider it' - you are. So talk it through with her, she probably has VERY different ideas as to what it would actually be like!