Fuck what others on the flight think of you. You won't see them again. Do whatever makes the flight as least distressing as possible for the children. If that's moving around and making noise, then do some of that.
No regard for the other passengers trapped in a confined space, then?
What do you think happens to autistic children with sensory sensitivities, exactly? I'll give you a clue... they grow up into adults with sensory sensitivities!
OP you've had some good suggestions here (like the wrapping up little toys one). Is your youngest still breastfeeding? That's by far the best thing, if so, for getting them through the pressure changes of takeoff/landing. I assume there aren't restrictions on how much formula milk you can take on, either?
Personally, I'd drug 'em, but that's just me.
I'd look into sedating antihistamines/cough mixture or something. Have you a sympathetic GP you could talk to? Some parents swear by melatonin for inducing sleep in autie/aspie youngsters, but I've no idea of the ins and outs of that.
A few suggestions...
Maybe try to get your toddler into a certain series/film/character and then load up an ipad/tablet with that.
Headphones if they'll wear them, with an audio story/music/whatever they like.
Something tactile like playdoh - that stuff was manna from heaven when (aspie) DD was little; kept her focused in public spaces for ages.
Take plenty of comforters (you mentioned a sucky flannel - have a few of those!). Have you tried chewy jewellery? Does she have any other stims apart from spinning (ie that she can do sitting down?!) that you can facilitate?