If the call to children() throws an error, that means the page('agenda') is not a page object. So check if that page really exists in /content, or if it is nested further down the page tree, you have to provide the complete path to the page: page('all/parents/agenda').
As for your first issue, without actually knowing what exactly you are doing, I can’t help.
Super late reply from my side, sorry.
I get your point. I decided to just have 3 separate installs. Because the controllers could not be shared (and yes, I could not get it to work ) I just update the templates by hand and have a shared css file.