Dont.
These themes don't (can't) hide children in the sense that they aren't in the arrange. The tracks simply get a height of 0 pixels so you can't see them. If you drag a marquee selection across those tracks all items will be selected and are subject to edits - even if you don't see what's happening.
Use the track manager or actions to hide tracks.
For example this macro toggles visibility (in arrange view) of all children of selected folder tracks:
- SWS: Save current track selection
- SWS: Select only children of selected folders
- Track: Toggle show/hide in TCP
- SWS: Restore saved track selection
It is also possible to build sets of toolbar buttons to show and hide tracks by name, eg "show only tracks with 'drum' in name" or "hide all tracks with 'drum' in name".
You can change any theme to do the zero height hack by opening the theme's rtconfig.txt file (it's in the theme's image folder, you need to use a zip unpacker if the theme is a *.reaperthemezip). Pretty much at the top of the file you find the line
Code:
tcp_heights 4 12 27 77
(the numbers are from my personal theme, will be different)
The first number (4) is the height of collapsed folder children. Try lower numbers, but as said 0 is not as good an idea as it seems at first glance.