It depends on a few factors... How severe is the leak/is it causing them to not use the kitchen as expected? Are you planning on getting this leak rectified within a reasonable time frame or is going to be leaking for a while?
If any items were damaged as a result of this leak, I would 100% tell the tenants to reach out to their renters insurance, assuming they have it. Maybe give a credit here if any items or foods were damaged.
Personally, I would say no to offering a rent decrease because of a leak. Not only does that change your income flow, but you also don't want to set that precedent for the tenants. Worse case, if you do the rent decrease, is that you have a non-cash flowing property/rentals below market rent as well as you have tenants who expect their rent to be lowered at every inconvience. You could suggest them to put their rent into escrow until the leak is rectified, but a rent decrease seems a bit excessive unless the kitchen is totally destroyed and or unusable.