Hi Dave
I wouldn't suggest anything hard as it will chafe her and cause
burns or sores on her ankles. When my daughter, Kelly's, right
leg would start striking out in very forceful thrusts, it would move
her all over the bed and tire her out. I bought one of those baby
neck braces sold in department stores. They are soft, covered
in terrycloth (washable) and use to help brace an infants neck
will be transported somewhere.
I cut a long scarf in half and sewed one half to each side of the
baby neckbrace then put the brace around Kelly's leg and tied
the scarf to the edges of the bed. During the day, when her
foot was real active, I would only leave enough slack for her to
move her leg about 6" off the bed. At night there typically
wasn't a problem so her foot wasn't tied down.
At first I felt horrible about having to use a constraint on her,
but it made her feel more secure in bed especially when someone
wasn't in the room with her. The baby brace never hurt her skin
either.
Hope this helps a little.
Love,
Jean