There appears to be some kind of bug going on while I'm doing this quest. Once I've used the ladder and started up the tree, the buttons to climb up/down are greyed out after the first two times going up. The dismount button is not greyed out, but when I try to click it, it says "You cannot do that right now." or something to that affect, in red text at the top of the screen. I tried dropping the quest, which left me in the tree and still unable to climb up or down. I was a bit panicky, because I couldn't get out of the tree, I couldn't climb up or down, and I couldn't throw the bear cub I had picked up.
I tabbed out to check here to see if anyone else had experienced this issue, but I didn't see any comments having said so. But when I tabbed back in, I was able to use the down arrow, finally. I was able to use it once, and then the buttons greyed out again. I thought "This is dumb... I have to tab out every time I want to press one of these buttons?" but figured at least that would mean I'd get the quest done.
Until I tabbed back in and the buttons were still greyed out...
So I tried clicking the quest giver to see if, by some weird chance, I could pick the quest up again while up the tree. I couldn't, but the buttons were clickable again! Once... sigh. Then I tried the same thing I just had- clicking the quest giver. And the buttons were clickable again! Now I'm getting somehwere!
So it turns out that, for some reason, the buttons were greyed out unless I was targeting something... Strange. I was able to complete the quest by having to target something each time I wanted to move up or down the tree. I don't understand why this is happening... I don't know if it's an addon I have installed, or my UI, or if it's just a bug, but if you're reading this, and having an issue with being able to click the buttons to move up or down the tree, or to throw the bear cub onto the trampoline, then try to target something and see if that will work.
Hope this helps!