child who just cannot sleep through the night

Hello. New user here so hope i am doing this right! We have a child who just cannot sleep through the night. She's 5, with ASD (adhd now being investigated) and a previous brain bleed at a very premature birth - all causing development and learning delays. She can settle, but she will always wake about 11-midnight and then it's all downhill from there. She takes time to settle, and is normally back up several times a night. We cannot (and won't) leave her to cry, crying results in vomiting within a minute in general. It always results in one of us sleeping on the floor. We've done all we know, we've kept a healthy sleep space since birth, we don't do television in her room. We tried melatonin fast release and it's made no difference. Slow release isn't an option as she cannot swallow the tablet. We considered a GloClock but she doesnt understand the concept. She has a lack of speech. I stress that this isnt so much an issue about us waking for her, but more that I worry that lack of sleep will have an effect on her learning.