Regarding your question; suitable for children this age (9 and 11 years old); honestly we have successfully organized another trip ( Manaslu Circuit Trek cross over the 5,160m) with Kids the same age as your children. You may see pictures of them too, but not in Upper Mustang. Upper Mustang is easier than the Manaslu trek and Upper Mustang Trek is good access to the road for most of the trip if anything needs help. So, according to their trekking routes, Upper Mustang is suitable for your family.