Ozangel I would not be able to tell you exactly what I would do unless I was there on the day witnessing how it was unfolding - but a suggestion I would have as a starting point is finding out if the crewmember who had experience with this particular disability could advise the rest of the crew on effective communication methods, and trialling those communication methods? I would also suggest the cabin manager/supervisor has a chat with the child's parents/gaurdian to find out if there is anything they can suggest for effective communication and reasoning with the child.
At the end of the day though, if I were not 100% satisfied that the childs inflight comfort and safety could be effectively managed by the crew, well offloading would have to be taken into consideration - obviously as a last resort, but a consideration nonetheless.