I have a keyboard!
I went for the first time last year with DD (then 7), friend and her DDs (7 and 4). Getting from car to campsite was HORRENDOUS. We knew the site was hilly but really weren't prepared for how difficult it would be getting our stuff from A to B. Plus we had no trolley and were about as far from the trolley hire as it was possible to be, so we'd used up our first half hour's hire just getting trolley to car (I think we had the trolley for something like 3 hours on the first day and 2.5 on the last - the guys hiring them took pity on us and undercharged on both occasions). This year my mate has an allotment and she IS going to buy a garden trolley and we ARE going to fit it in my car, if it kills me.
Normal camping was fine, even though we were on a hill, but we were going to go for Camping Plus this year for certainty of pitch and being closer to the car. However the cost went from 10mx7m for £120 to 5mx7m for £100 - she has a 4m bell and my tent is over 3m wide so we just cannot squeeze onto one pitch, and we can't afford £200 extra. We didn't use showers - lots of baby wipes, and I washed my hair in a bucket. Loos were occasionally dodgy, but more because if you have 30,000 people using a series of portaloos, some are going to get nasty, rather than poor maintenance. My only proper festival experience prior to this was Reading in 1994 and I STILL remember how bad those were.
Sleep was no problem. We were shattered. I took earplugs, for some reason they worked brilliant for three nights and did nothing on the last.
We took a picnic tea for the first night, and some snacks for the rest of the weekend, but mainly ate out. It did add up but I really don't think the prices are as outrageous as people say (I eat out quite a lot, but my friend less so, so I don't think it's just me being oblivious). There were kids portions from the fish and chip van, Pizza Express pizzas (could be shared, or not, if you're DD), the Mash Shack was definitely kid friendly. DD is highly fussy and I didn't have trouble finding her something to eat (um, although she did have strawberries and marshmallows for lunch on Sunday - long story). Some of it was really good - I have very fond memories of a burrito (such perfect festival food) and a Thali Cafe curry.
Fancy dress I couldn't see myself ever doing, for similar reasons to the not cooking and taking loads of food - we didn't take everything but the kitchen sink, but still had SO MUCH STUFF. Practical clothing, definitely, not fancy dress! That said, DD ended up spending most of the weekend in fairy wings and fluorescent pink false eyelashes. I reckon you can have fun dressing up without it actually being fancy dress - my observation is that people at festivals dress like toddlers, social norms go out of the window, you wear what you fancy. Fairy wings, jeans, wellies, tutu, t-shirt and sequinned waistcoat - perfect. :)
Oh, and I am definitely going to be sidling up to five this year for her resident's tips on how to get in without sitting in the QUEUE OF DOOM.