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.