Had to work with a python programer on a small java project (in uni). I passed some (handcrafted) strings in an Optional to be explicit an first thing he does is check whether they are empty (sending on empty strings would not have been problematic). Also he had compilation errors on his branch that lasted over a week. What python does to someone.
If you really like apple juice get the homies together, fill up a trailer at an old apple alley or orchard, rent a trailer sized press and everyone goes home with a trunk full if juice.
I once saw a human look both ways before walking across the street and I thought "Wow. What a dumbass, he could just walk a mile to the unterpass and not have to worry about cars."
"When the patient woke up, his skeleton is missing, and the Doctor was never heard from again! [laughter] Anyway, that's how I lost my medical license" - Medic from TF2
It really depends how your tunnels are built, but ropes going in different directions are probably a good obstacle that could be removed from the outside by untying some simple knots in case there is a problem inside.
Anyways, here is what I typed out before I noticed you wanted to build tunnels:
I think a small bridge made out of a wood frame and a net could be interesting.
By having the net hang in a U shaped cross section the kids can't fall of to the side, while allowing you to help out from above.
If the net is hung loosely it softens falls and keeps it a challenge as they grow, but it might be difficult to traverse for small children.
If you want it pulled tight I recommend some parallel ropes at the bottom to walk on.
Also a tilted plane with wooden holds or a knotted rope.
Had to work with a python programer on a small java project (in uni). I passed some (handcrafted) strings in an Optional to be explicit an first thing he does is check whether they are empty (sending on empty strings would not have been problematic). Also he had compilation errors on his branch that lasted over a week. What python does to someone.