I marked it as not made for kids, like usual. Plus I have made countless films featuring my children in the past but never had this problem before. Taking a quick look at my films of the past i can see that the children have appeared on a bunch of thumbnails but comments were not disabled.

Three times i have adjusted the settings to permit comments and three times it has reverted back to no comments 30mins later.

I will try changing the thumbnail now and see if this makes a difference...