Here's the deal. If you agree to any financial exchange outside the courts, you will get those phone calls on the house anytime anything goes wrong because they will always blame you for lying to them.
If you are being honest and sincere about water problems only being present from the sprinkler system, and it seems you are to me, then you were honest and forthright with the buyers and would owe them nothing.
Paying them any money may compromise your earlier disclosure and your reasons provided. It is kind of like admitting you lied.
If you were honest with the buyer, tell them you were honest with them and explain to them that you can't compromise your earlier honesty by agreeing to compensate them because then it would make you look like a liar, and let them know you've consulted with a professional in the matter. You can be friendly in all of this, and tell them that you feel bad that it happened, but that you can't accept responsibility for heavy rains.
On the other hand, if the den leaked a few times on heavy rains when you owned the place, you blew it and failed disclosure under the new laws of the land. Kind of a little Enron type deal, and you really ought to pay for the fix (under the new laws of the land).
The cost of setting drain tiles and a sump for this room might be very expensive, like in the thousands of dollars. I'd agree to nothing. Sloping the land properly might be another less expensive solution.
At any rate, I wouldn't pay them any money outside of legal channels, it is a sad, but real fact with resolving these types of issues.