Well its a good thing in terms of the cycle, but as you know its an extremely bad thing for the fish, I hate to say it, but be prepared to lose some if not all of the fish. Can you increase aeration into the tank, either via an airstone or by the filter outlet to break the surface of the water? If you can, do it as soon as possible. Nitrite binds to the oxygen carrying component (haemoglobin) in the blood, symptons include gasping and rapid gill movement, on top of the ammonia poisoning they have already suffered, your fish need to get relief as quickly as possible.
Is there anyway you can re-home these fish temporarily? Ask your LFS or someone with a tank? Just until the cycle is complete?
In the meantime, water changes and plenty of them, also the nitrite stage of the cycle usually takes twice as long as the ammonia stage of the cycle, therefore you will doing this for many more weeks to come. If that nitrite does not come down with 50% start with 75% and keep going as often as need be. Good luck, keep us informed of your progress