Anxiety and Sleep Issues

Most likely their strengths are their incredibly vivid imaginations. This means they might be able to envision blueprints, envision and measure to an exacting measurement. Envision maths or chalkboards of formulas. But when young this wild imagination, rather than being a powerful tool crafted to excellence as a mature adult in their 40's or 50's might, is out of control and learning to discipline the mind might need a spiritual guru and a good deal of wisdom. 

Every coin has two sides: Excitement IS Anxiety for a young autistic human who is monotropic. Hyper-Focused Attention to detail IS a type of Aloofness and can miss any not inside the parameters. This incredible imagination which might be capable of Creative and Technical crafts IS a alive with other hauntings and imaginings which might have made Steven King jump at night. 

I'd suggest for start be Very Careful what you allow them to be exposed to in media. Read classic children's stories and allow them to create their own images, which will be far less fierce than what's on tele. Perhaps classic stories on podcast or soothing music at night. Ideally, meet your kids where they're at and allow them to feel reassured that if they need you, you'll be available, which is really your job for now. I'd sit in my sons room and read - either to him or myself as he fell asleep. I personally think the ideals of society - forcing children to feel alone and isolated to toughen them up doesn't do this in the least. It severs their trust and creates a sort of hostility and resentment which takes on a form of being 'tough'. But it makes everyone's lives miserable. Children should be helped to become themselves from the inside out. First help them to feel safe, wanted, appreciated. Do whatever you have to to earn their trust. Treat them with respect and kindness. Perhaps they don't learn to drive until 20 something, but they treat others with dignity and work diligently. Matters of the heart are the most important. Creating a secure human who feels protected, is a human who won't need therapy at 40 after a terrible marriage and a draining divorce. 

So, sit with them. Pour yourself a glass of wine, put the phone away and indulge in your favourite author every night. Children give us new operations for a more enjoyable life. 

You may have tried this, in which case apologies, but are there ways they can be in control of their environment at night that might reduce anxiety? Like fairy lights or night lights they can control from bed, a player for music or audio books (something like a Yoto player if you don't want tablets etc in the room) and maybe a weighted blanket for security?

Also when my kids wake in the night I always go to their bed rather than having them in mine. It's a squeeze, and they're both younger than yours, but it does keep our bed for us.

I have an Autistic+ADHD daughter, and I’m also Autistic. I can appreciate the difficulty you have, as I’ve experienced it myself. However, putting the child to bed, and leaving them is good advice. It does work in the end, but it’s a very long and hard road! I can’t stress that enough.
I used to make sure she went to bed at the same time every night, and with the same routine. I would leave the room. If she was stressed or restless, or crying or whatever….I would go back, lay her down, say the nighttime phrases and go. Believe me when I say I was up and down all night to early morning sometimes. Persistence paid off.

All children are different, and some parents feel unable to leave the child. I know too many parents who’ve held, rocked or slept with their children, or allowed them to get in bed with them. It may help temporarily, but it’s going to make things harder in the long run.

Hopefully someone else here will give you some more options to try.