The pass for guests with special needs/disabilities is FANTASTIC it allows you access to the rides without queueing, and means you can use the special roped off viewing areas for the parades too, which don't get as crowded.
I took my DS (also autistic) last summer and it was a big gamble as he'd never been on a fast ride before, and is scared of dressed up characters. However he's disney mad so I took the risk. He went on every fast ride he was tall enough for, and met lots of characters. We saw most of these at a meal in Cafe Mickey. As 'Eeyore' came round a very loud happy birthday song started and DS started to panic. 'Eeyore' noticed, came over, and put his 'paws' over DS's ears. 
If both of your girls have autism I would do it so you each take in one DD each and get a pass each, as although on the rides you could all get on using the one pass, in the parade viewing areas they were very strict of only letting in one additional person with the pass. If you had one for each DD you could all view the parade together. Take a DLA letter/ social services registered disabled person card/ diagnosis letter for each of them.
If you go to watch the Cars Motor Stunt show, this also has a special viewing area if you have the pass (as do the other shows in the studios park), pre-warn your DD's there will be some stunt fire near the end of the show, or be ready to reassure them. I didn't know about this in advance, DS panicked as it came up quite near the viewing area and he was really thrown by it!
Hmm... what else? The portions in the 50's diner are huge, share. There is a McDonalds if your kids want something safe and familiar but it's very busy, we bought and sat and ate it outside with space and fresh air. Pizza Planet has kind of a pizza hut style buffet. This was good for picky eaters, and had a play area so you can sit down for 5 mins watching while they burn off some steam. I took some ziplock plastic bags to breakfast for croissants, rolls, etc to snack on but I realise some aren't as shameless as me.
I also took a bag of apples and some cereal bars from home - if your dds are picky eaters like my DS, then having some familiar food is reassuring.