It's worth going both days, there is no way you'll be able to see everything you want to in one day. Also, take waterproofs and wellies. If its wet it turns into a mud bath.
A tarp or waterproof picnic blanket is a good idea for eating and watching some of the displays.
Oh and money. You'll be able to buy re-enactment quality tat, as well as the wooden swords and shields and dressing-up tat you normally find in EH shops! Plus there are usually some nice "medieval style" alcoholic beverages available. It's where I acquired my mead addiction....
I haven't been in a while (wanted to go last year but it was flooded out and cancelled) it was always a great event, lots going on, loads to see and do. You shouldn't have any difficulty taking your dog, so long as they're ok with loud bangs (military displays tend to be load) and loads of people!