My DD is 7 so your DD is a little older and will have a bit more stamina perhaps. However, I still would recommend only getting a daytime pass.
If you look at the day and evening timetables, much of the show parts are repeated in the evening. So, the only bits you really miss out on are the additional competitions and rounds. It is not a big deal missing out on the final rounds as they make a huge fuss of every round, with plenty of pomp and fuss - flowers, rosettes, dramatic music and lighting, regimental blokes in shiny uniforms and tooting fanfares etc. Honestly, it is a huge spectacle.
While you have your allotted seat in the main arena, you are free to come and go between the arena and the shopping area. This helps break up the day and means you can skip a few of the less interesting comps and enjoy the shopping area. At HOYS this year, there was an area full of different horse breeds to see and pat/stroke, a big X Box "pretend you are riding a horse in the grand national" type thingies that look like a plane cockpit, fairground horsey rides, endless horsey shopping from chaps to horseboxes etc etc. Good mix of specific horse owner stuff and pony mad girly clutter. Same deal with the British Open - full of horsey clothing, books, toys, etc etc. and the odd freebie bag..
Oh - and personally I don?t think it is worth paying for the really expensive seats. The next bracket up, somewhere in the middle of the numbers, is just as good if not slightly better. For a start, the aisles are shorter so fewer people pushing past! Also, a little higher up is a bit better for viewing too. The first year I bought seats right at the top. They were ok as you can still see and hear everything. However, definitely enjoyed it more in a central mid height position this year.
Ok - will shut up about it now