Keep in mind that there is a lot more that factors into restrictiveness than chain length. Maximum knee separation and stride length might be better ways to measure that.
Also, I feel like at least in public, some range of motion is helpful for hurrying and handling obstacles and what not. If she already struggles with stairs now, she should definitely get more practice with that before shortening the chain.
Whether it makes sense to wear more restrictive chains in situations with high risk of cheating and low mobility demands, like the privacy of her home, is a different matter. I think giving her the ability to restrict herself if she is tempted to cheat is a good thing, with no punishment except for the fact that the restriction will stay in place until she has good reason to need more mobilitx again.
