Character mistake: The wolf-man says, "I'm the guy they shot last night," and Del's partner says, "the van crash last night," but those events were two days prior to these scenes.
Character mistake: It doesn't occur to our heroes (admittedly kids) that you don't need to understand German at all in order to read it out loud. It's arguably easier to read than English! The only inscrutable letter would be "ẞ" but Scary German Guy's right there for help. (True, this is essentially what they end up doing in the end, but they still come about it through a "you don't need to be of age to be a virgin" realization rather than the even simpler "you don't need to know German.").
Answer: It's unknown how the dog got into the treehouse. After the boys put their hands on top of each others, the dog puts his paw on and Rudy questions how the dog got in with none of the boys being able to give an answer.