Outdoor session climbing real trees in a controlled manner.
Some suitable trees, adult helpers to supervise (one per tree)
You need some trees with sufficient low level branches that can safely bear the weight of your beavers. Each tree needs to be assigned an adult to supervise the climbing: encouraging the timid; coaching those that haven't done it before; and catching the overly bold before they fall.
One at a time (per tree) each beaver should be given an opportunity to climb up to a level where the adult can safely reach should they require assistance to return to the ground (or support to prevent a fall).