If you don't know whether or not their mother is around, you should not do anything.
If you do know that she is around, and she seems to be nursing, you should not do anything.
If you cannot legally access their property, you should not do anything.
Kittens can often look quite scrawny while still being healthy. Do not, under ANY circumstances, give them any milk or anything whatsoever until you know for an absolute fact their mother is not nursing them. If they're under 8 weeks old, or do not have all their teeth in, they'd need KMR Kitten Formula(I usually use the canned sort, but the powdered formula works just as well) until they were old enough to move up to wet food. Do
not give them dairy milk of any kind, ever - even in a state of starvation, it will do them more harm than good.
The real caveat is that once you start feeding them, they will become dependant on you. So, unless you are fully prepared to handle the responsibility, and know for an absolute fact that these kittens have no mother or food source available, leave them alone. I've been rescuing cats for about ten years, now, and I've hand-raised and bottle fed half a dozen litters in the last two years alone. It is not easy, it is not cheap, and it is not something that someone with a weak heart should really volunteer for because, even under the best of circumstances, kittens are prone to dying for no medically explainable reason.
Current cat population on my property is in excess of 50 outdoors(around 28 of which were born this last mating season - they all get to a certain age and then leave, aside from the breeders), and 5 indoors. I actually just got done worming some timebomb kittens today.
It's difficult to say to anyone, but quite frankly, if you don't know what you're doing, you're more of a danger to those animals than any natural force. There are hundreds of websites and resources you can use online, though, and I'd suggest using google if you really are quite insistent upon doing something.