I am sorry for your loss of Bala. So young, and sounds like very sudden. That is so hard to deal with. My condolences.

Now for the kittens, I'm not sure of their age at this point. Kittens under 4 months can test false positive. Here is a link to a good summary of FeLV:
http://www.catchat.org/leukaemia.html

Since the Mom is negative, the chances of one of her kittens have it is slight. Retesting in 12 weeks is the next step.

This is a decision only you 2 can make, once you have more information. You should also call your vet practice and get information from them.

Do let us know what happens.