It’s not a feature of “sideMenu” it’s part of the uiRouter documentation
And it doesn’t mean it can only have one child state, you can have others but it’s just there is a link between the state name and the view that the template will be inserted into, need to think about the dots very very carefully.
When you wrote:
You are telling uiRouter that you want to insert the template subpage.html into the view named menuContent in the state eventmenu.attendees (it’s parent) BUT the view named menuContent isn’t in the eventmenu.attendees state it’s in the eventmenu state (it’s parents parents state) so it can’t find it.
The easiest solution (it’s not the only one!) is to just change the state name from eventmenu.attendees.subpage to eventmenu.attendees-subpage so that the parent state is eventmenu and so uiRouter can find the menuContent view.
If you read through the docs i posted above though you’ll find there are quite a few other ways to config uiRouter but i reckon sticking to the above will avoid a lot of pitfalls.