To enter the tight passage is an Escape Artist check (DC 15). The side passages 10-feet beyond the opening are tougher to traverse (DC 20). The PCs can choose to take 20 but it will be very noisy and take considerable time to move into the passage.
I'm not sure if Aid Another should apply to a Escape Artist check, but we'll say it does. But you can only do it for one PC. Trying to help multiple characters through is to difficult due to the tight spaces.
Initiative Order (1st Round)
Quinn: 20 (readied action)
1st Group of Dire Rats: 17 (entangled, as per web spell's description)
Rat, Normal: 13 (entangled, as per web spell's description)
2nd Group of Dire Rats: 11 (entangled/immobilized)
Aureus: 10 (total defense)
Caerth: 9 (readied action)
Rat Swarm: 5 (entangled, as per web spell's description)
Phar: 4 (readied action) Dire Rat: 4 (entangled/immobilized)