June dear, please do not cry ... (although I myself cried as I typed the previous post and this one too) ... and please DO NOT blame yourself ... it is one of those things that happen in a life's journey ... theirs as well as ours ...
The virus can survive only about 2 hours in a dry environment, and about 48 hours in a damp environment (such as a litter box), so June, go about life as you have been doing all this months and years with the kids ... your house and you are clean enough and all the cats have already been exposed, some may even have developed partial or full immunity. For this reason also, DO NOT rush out and attempt to vaccinate your cats.
Here is some important findings on the FeLV vaccination:
Injection Site Sarcomas
In recent years, a disturbing phenomenon has been identified by veterinary researchers. A relationship has been found between feline leukemia virus vaccine (as well as rabies vaccine) and the development of very aggressive tumors at the injection site. The numbers of cats who have developed these tumors is very small compared to the total number of vaccinations given (1-3 per 10,000), but the outcome is devastating for cats that do get these cancers.
Several veterinary schools are actively researching this problem. It remains unclear exactly why some cats develop tumors in response to vaccination; a genetic predisposition is thought to be part of the problem. At this time, feline specialists are recommending that leukemia vaccination be reserved for cats that are at risk for exposure to the virus.
Strictly indoor cats generally do not need vaccination against FeLV. We can advise you about exceptions to the routine vaccination protocols.
(
http://www.greenbriervet.com/newsletters/felv.htm)