I'm currently working with a customer where there is a lot of pages added to one parent page.
Into one page there is about 100-500 child pages added every day.
Is there any way to prevent child page from being shown in the editors treemenu?
There is no standard way of doing this, and this will lead to big problems down the road.
When you expand a menu item in the tree, EPiServer will load two levels down, to give you the +/- symbols to indicate if there are any child pages. I do believe that this has been subject to optimizations, but I'm not really sure what has been done.
Never the less, having so many pages on one level will give you a usability problem real quick - as you've probably discovered already. The best solution to this that I can think of is to organize the pages in child containers, by date for an example.
\News
\2007
\January
\01
\02
\February
\01
\02
\...
Your templates does not need to expose this structure if you do not want to.
/Steve