The Category class (app/core/code/Mage/Catalog/Model) has a getChildren method which returns a comma separated list of category ids of child categories.
I’m trying to call this method on a category but I get a ‘cannot convert tree node to string’ error.
Sounds like you either tried to save it into a set string variable, or u tried die($returnedValue). if so try vardump($returnedValue); instead
Hope it helps =)