You started with immediate action, but then sabotaged the urgency and tension by over-describing (bare, whip-like branches...tiny grey puppy, Becky...thick hemlock branches...steep rocky slope...).
It's not that there's anything wrong with those phrases in themselves, but Edmund appears to be running from danger and probably isn't thinking in adjectives.
Also, you've got one of those "as" constructions, which seem to be ubiquitous among learning writers. The problem is, they often put the action out of order. In this case--
their bare whip-like branches leaving stinging welts as they slapped across his face and chest.
--we need to see the branches slapping his face and chest before we're told they left welts.
There are ways to rewrite that convey the same information but without adjective salad. You can say the branches whipped across his face. You can stay he forced himself through a stand of hemlock. You can have him holding a tiny puppy and introduce her name later.
Thanks for your help!