Either way, depending on the contract, it will cost you money. eg.
Pay who is on leave and pay the subcontractor to deliver on schedule = happy client.
Deliver late offering discount to make up for the money the client will lose due to the delay = maybe not so happy client.
The best advice is to be completely open and honest with the client. Explain the reason for the delay, and the steps you propose to take to deal with it. Above all, be prepared to agree a new delivery date that you can expect to meet.
You say “The reason behind this due to unplanned leaves of resources”. That means absolutely nothing. Don’t even think about using a phrase like that when talking to a client. You really have to explain the problem in plain English (or whatever other language you use to communicate).
You’ll find that clients hate having their project delayed. But they hate it even more if the contractor doesn’t warn them of the delay, doesn’t tell them the reasons, and is not ready to agree a new date.