If they lick it off another kitten or somehow get it in their mouths, they can start to foam, but foaming can also be a sign of poisoning. I seriously recommend that you call an emergency vet right now and talk to them about this.
I applied Frontline about 35 minutes ago. He is 9 wks old. Is that to young?
My Annie would slobber foam EVERY time I applied Frontline. In fact, the vet told me to possibly expect it before I used it the first time, and that it would pass in a few moments, and was not harmful. It worried me, but she never had any other adverse effects from Frontline.
I've had this happen a few times after flea treatment was applied...I have 5 cats to treat and I try and do it now late at night when I know they will be less active and I have even done it when they are sleeping...less of a chance they will lick at it on themselves or the other cats..