I put this here because some families have found success using these and they are frequently used for children with Down's Syndrome. They can be made by handy sewing people or purchased at http://www.hiphelpers.com.
As I understand it, they are used to help pull in the child's legs and counter that froggy look that low tone kids have. They should be worn when the child is awake and moving. They can promote trunk rotation over the pelvis which is needed for eventual adequate weight shift in the trunk when walking. They should be worn tight enough to hold the hips shoulder-width apart without causing any red marks or constriction.
We just tried hip helpers on my boy. He is 7 months and is still a bit froggy. He did great with the hip helpers. We opted for ones that are somewhat loose. They aren't in the least bit constricting but do seem to provide input through the hips, which I think he needs. He seemed to like the hip helpers and they seemed to make movement easier for him as opposed to more difficult.
In the hip helpers, he was able to roll over from his back to his stomach and keep his knees under him (in preparation for crawling??). Even if your child is just borderline froggy, they might be worth trying to see if he or she moves any differently or better in them.