As the servant of one cat (previously two, the other being an "ex-feral" which managed to tame us, til it died - at 18+ - last year) I think your wife's reaction IS very OTT. Although they both suffered very occasional bouts of fleas, they were quickly cleared up with flea treatment on the cats, but little or nothing on the house. Another good ploy is a flea comb (bit like a nit comb). Used regularly this will show both fleas and "flea dirt", which can indicate the presence of new fleas. That said, I'm not sure a small kitten is a good start for a 4-year old. Without prejudging your wife's son, they can often "over love" a small kitten, which may then get fed up and react badly to constant attention, however kindly meant. Something like a hamster or goldfish, or even a budgie, might be more appropriate.