6yo pre verbal daughter won’t sleep

Hi

My daughter is 6, has autism and has severe learning difficulties. She’s never liked sleeping, but recently it’s gotten out of hand. She won’t stay in bed, getting up just seconds after we get her back in bed, and she runs straight to me and her dad, wherever we are. This happens over and over for hours - in the last week we’ve had several nights where she’s had only 4 or 5 hours sleep because of this. She knows she’s supposed to be in bed, and I’m worried she thinks it’s a game. She’s not distressed or upset at all - she’s usually smiling in the cheeky way she does when she thinks she’s getting away with something. 

We’ve tried everything - night lights, blackout curtains, total silence, white noise machines, a strict bedtime routine, and she has a prescription for melatonin. Social stories don’t work because she can’t understand or pay attention to them. Nothing works.

The only way I’ve found of getting her to sleep at a remotely reasonable time, is to stay in her room with her, cuddling her lower legs so that she stays lying down (a normal cuddle doesn’t work because she kicks and flails wildly with her legs, not in an unhappy way, it’s just something she likes to do). That way she succumbs to sleep within an hour because she’s forced to stay still and calm. However, the problem with that plan, is I’m pregnant, and in a few months I won’t be physically capable of doing that anymore (and for some reason that strategy doesn’t work if her dad does it).

Does anyone have any similar experience, or have any ideas for what might help?