Sorry for the loss of your kitten and hope the other one has recovered
I would advise to only use vet prescribed treatments for anything to do with cats and dogs
Sadly people buy this type of thing in the pet shop and it proves to be very costly false econonmy with tragic results
My neighbours cat was left blind in one eye due to the same product - she now only uses vets stuff or comes to ask me for advice - we have an awful lot of cats and only use vet prescribed products