From my experience, the more you insist they participate, the more ground you lose. Encourage, but don't push too hard. They will do it when they are ready and not before.
Bear got praise heaped on him from me for being willing to take off one shoe for about three seconds during a class recently. The praise occurred as the shoe came off and after it went back on with no negatives for it going back on. I don't expect them to come off, but if it happens I let him know he is making progress. I know that if I expected the shoes to come off (like I did the first two classes) he would NEVER have gotten to the point of taking one off for even a couple of seconds.
It's really a comfort level thing. I mean I would honestly never choose to jump out of perfectly good airplane, but some people do. It is far beyond my comfort level. There are people out there who are probably very confused by my hesitation. However if I had enough time and enough training I might get to the point where I was willing to try, but only if it was my choice...
On the other hand, I am very at home with rats and snakes. Put my in a tank with pet rats or non-venomous snakes of a non-man-eating size and I won't bat an eye. I know of other people who would probably pass out at the very idea of such a thing. However given enough time and the ability to control it (no outside expectations) and they might get to the point where they would consider it. The thing is I flat out don't understand what scares them so badly...
In some ways that's basically what we are asking of our extremely sensitive kids. They don't have the life experience to know that odds are it will be ok and other people will not try and eat them or anything else untowards to them. As long as they have the time to adjust, however long that might take, and the constant non-judgemental support of their parents who truly believe that other people are not all that horrible they will eventually start to relax and explore.