I second taking a duvet. We take two- one for on top and on bottom.
As for camping with babies we always take a travel cot or the carry cot to a pram and treat bedtime as normal. We have been to a few family friendly festivals and they have all had good facilities, the only issue are the cues to use them. We always take a bucket to fill with water to take back to the tent to wash the kids down with at bedtime so we can get their pj's on them straight after and into bed, instead of dragging them back to the tent to do the whole routine.
Do you bottle or breastfeed? If you use formula than I would buy the ready made cartons of formula to use to make life easier.
Id also recommend using a sling (if you dont already), especially if it is muddy. I always take my pushchair for just incase it is hammering it down, but for the most part my kids would be strapped to me and it just made things so much easier- especially trying to fight your way through crowds as it is bad enough doing that on the high street let alone in a massive crowd at a festival.
I cant think of anything else at the moment, so if you can think of specific questions let me know.