If you've been caring for them for the past few years then they are no longer kittens but fully grown adult cats. Are they vaccinated and neutered? Can you not take them with you? It's very difficult to rehome adult cats unless they are very social and have up to date medical records.

Putting them back on the street would be extremely cruel as they would either be hit by a car and killed or die from starvation or disease.

Why not look at taking them with you. They are attached to you and obviously you are to them or you wouldn't have kept them for so long.