I found a the solution.
In the rules section of I have>
Post Type | is equal to | events
Page Type | is not equal to | Child Page (has parent)
I used “is not equal to | Child Page (has parent)” because this will recognise returns true all root events. If I had it as “is equal to | Parent page (has children)” then it would would not return true when I had a root event page with no children.
