@Sherif30 Airbnb has its own process for requesting compensation for damage; it doesn't automatically assume liability for every repair that's needed during a stay.
In your opinion, was this damage caused by misuse or negligence? Would you be comfortable requesting that your guests pay you for it, even though they commendably reported it to you right away and don't believe they are at fault? If so, the procedure linked above is an option for you (though to be honest, it's quite a long shot). If not, you're best off considering the repair to be a normal maintenance expense, and thank the guests for reporting it.