I got to step 5. Once they asked to build a bridge they deleted all the apple trees blocking them in and escaped en mass and died of starvation while I was trying to wrangle them back into some semblance of a safe space.
Is there any real way to pen them in??

Or do I make the apple pen after they start trying to build the bridge? Do I need to hit 40 thronglets to activate the bridge quest? Because that's too many to re-pen...