Some Airports (Manchester and Stansted - I know of) have support to help children/adults with Auitism through the airport. Think its different for each one but worth looking into for your airport. Might ease the process.
Give yourself loads of time. Clearing security can take forever - also triple check your bags for stuff security might take out. I got caught out by a bottle of hand sanitiser in our changing bag I put in a pocket I never use/forgot about. My NT 2 year old found it quite upsetting seeing our bags being taken away to be checked again.
What does help keep her calm at home - if even for 5 minutes.
What about a find me bottle - lots of small things, pennies, plastic toys, paper clip, etc in bottle filled with rice/couscous etc and they have to try and spot as many as they can.
Do you have a sling for your little one - we took my son on holiday back in May when he was 10 months and the only way I could get him to settle was to be in the sling on me. As soon as I put him in it he fell asleep - try and get the seatbelt round them in the sling and they they can stay where they need to be.
Hopefully others will help you out - I would ask if there was anything I could do if you were on my flight.
Good luck.
Oh and my other tip - hand luggage as rucksacks - we took holdalls and really regretted it.