If they are black and white.....they are domestic rabbits. Wild rabbits appear totally black for the first couple of days and will start growing their brown fur almost immediately. Wild rabbits will have a full coat of brownfur within 5 days. They are usually moving around and exploring by seven days. Their eyesopen aroundeight days old in most cases. Wild rabbits only feed their babies once or maybe twice a night and it's usually just before sunrise. Wild rabbits also have a much quickerand violent weaning....and that is when most rehabbers lose cottontails (and why many refuse to take them). This has the classic signs of a domesticbeing abandoned...or "set free".
I truly doubt that a wild rabbit would foster domestic babies. Wild rabbits are too busy with their own litter every 30 days or so. In lab tests, native rabbits could impregnate domestics, and vice versa, but the embryo died after the fourth cell division. But...I would consider those tests unreliable since there isn't much of a track record on this....so there is still questions about this.
Can you post some pics.....and maybe snap one of the mom? It seems since they are fat that mom is returning....but if a mom is showing up during the day, I can assure you that it is not a wild rabbit. It would be rare that a true wild rabbit would expose themselves during the day.....not totally impossible but rare.
If these are domestics, I would suggest trying to secure the mother and bring the babies in with her. If they are wild, leave them alone. Wild rabbits are the most difficult species of small mammals to raise in captivity, they usually stress out in captivity and it's illegal in many states to be is possession of native wildlife without proper permits/license.
But some pics would help to make a certain identification.
Randy