Festival loos are something that all non-festival goers cite as reason they can't do festivals. And I'm sure some festival goers have horror stories too (I know I do) but with a little planning and some ability to be a bit more relaxed around poop than usual (more parents can cope with poop in my experience) the festival loo experience can be fine!
Camp Bestival has two types of loo (well actually it has three but the last I haven't experienced...more about them later)
The first is the old faithful, the portaloo. Most festival have them cleaned and emptied fairly regularly - don't camp too close to them or the 4am hose down and pump may wake you! They can get gruesome but it's down to the festival goers themselves being disgusting and drunk rather than the toilet itself! They will run out of toilet paper and of hand sanitiser. Take your own of both and wet wipes too - make a small bag for toilet trips containing the essentials.
The second sort is the 'compost' loo. They are a variant on the long drop! They have steps up to them and no water, under your seat is a 'pit' filled with sawdust. You take a cup of sawdust from the bin provided and chuck the sawdust down to cover your poop! They are less smelly than the portaloos (probably as they are very open and airy) they have no chemicals, but you do see a lot of poop. (Kids love this) They tend to stay cleaner. You may still need your own toilet paper and hand sanitiser.
The third and more mysterious option is the luxury loo - Camp Bestival has a secret land of luxury pooping that I have never experienced...