Is it possible that the child objects are at a depth greater than 20?
Something else to look at... using the Object Spy, spy out one of the items you are hoping to find and verify that they do, indeed, have a property called "tagName" (I'm assuming it does) and, if so, that the value of tag name is, in fact, what you are looking for.
I'm not familiar with EvaluateXPath so I can't say for sure if they are equivalent expressions but I'd start with Object Spy and make sure of where you actually find the items so you can adjust your FindAllChildren parameters.
