Take the child`s pelvis and help to guide it back. Look for small variations, sitting first to the easier side, then to the other.
You can always switch back to rotating the knees like you did in the beginning, and see whether it gets easier to sit down.
Now if possible put the hands down as well and start to take the pelvis back, a little bit right and left (not all the way to the side yet).
Go very slowly. You want the spine and ribs to get softer and relax. A falling or frightened child will contract the muscles.
Put the child`s upper body on the roller and gently pull one leg back.
If the child is able to stand on all fours, gently and very slowly help it sit a little bit to one side, then to the other, without lifting the hands off the floor. It is more important that you support the pelvis and help the spine get long than getting all the way down to the floor. Think of making the movement gracefully and beautiful rather than big.