you may be able to line something up with your local rescue (don't tell them the whole story, but just that these kittens are in your garden, you are happy to keep feeding them but would want them removed in the longer term. That way, if you keep nagging them, they will come get them when they have a spare run or fosterer. If you were to take them into your house, you could keep them in a room separate from your own cats (I would do that anyway to reduce stress and cross-infection) and they would be safe and dry and warm ...