Yes, if you are 100% sure it's cat flu, ring the emergency vet NOW. My two long-ago cats had flu when they are were about 2, they were very sick.
You really ought to have asked for cat flu injections with their annual injections - it spreads like wildfire, you need to get your other cats looked at too, and isolate the kittens and their mother now - but please ring your VET now, the emergency number will be on their answerphone.
BTW who made the diagnosis of cat flu for your girl cat and the other two? Surely if it was the vet, you would have mentioned all the others, and particularly the vulnerable kittens?