Sadly, a dog with a prey drive is not going to be trustworthy with the other kitten, so you'd have to be ever vigilant about never letting them be together. As you have seen, it only takes a second for tragedy to happen, so even if you are in the room, you likely would not be able to prevent it.

Maybe you should, sadly, consider rehoming the kitten? You've had the dog for a long time, but the kitten may adjust to a new home just because it is so young.