I have a 3.5-year-old daughter. She doesn't know how to turn over in bed. I myself know how to, but not how to teach how to do the same or in an easier way.
How do I teach my 3.5-year-old how to turn over in bed under her blanket?
Young children tend to sleep a lot more deeply than adults, so I don't think this is something you can teach them. Even if your child learns how to do this while she's awake, she most likely wont be able to do it in her sleep. We just try to always use very light (but warm) duvet's rather than much heavier blankets. This tends to help prevent them getting kind of tangled up in them. Also keep the room warm enough so that the covers don't need to be pulled all the way up. We try to make sure ours are only covered up to their chests. They figure all these things out in time on their own. Until then just check on them occasionally through the night and they'll be fine.
I check on our kids every night before I go to bed, at which point I sometimes find them under the blanket, but have found various combinations and placings of blankets and children. They have been cold at times and lie sleeping curled up not under the blanket, and I tuck them in. However if they get too cold they will wake up and go back under the blanket. I guess the point is that mine are up to 6 years of age. As far as I'm concerned, knocking the blanket off is normal. Sometimes they will kick off the blanket because they are too warm; I know mine do as when I try and put it back it's immediately kicked off again and the child is still asleep!
If they are disturbing you lots at night, then warm pyjamas is the correct solution. When asleep, they could be doing anything.
We have in the past tucked another blanket over the top of the existing one and then firmly underneath the mattress for when it is really cold they can't kick off their blanket, but you can't do this when it's not cold else they'll wake up cooking.