Stray Cat Alliance has very specific advice on what to do. So check it out.
Assess the situation. If they are in a safe place (not in the middle of the street) and look fat and healthy, the best thing is to leave them there. The mom is probably foraging for food.
Leave wet food and water by her babies. Try and wait as long as you can to make sure Mom comes back. If you start to hear the babies crying, wait a bit longer as Mom will probably hear them too and come back.
If the babies are cold, or getting cold, put uncooked rice in a sock and heat it in a microwave. Put it around the kittens, making sure the socks are not too hot. Make sure the kittens can crawl away if they get too hot. Rice works better than water bottles because when it cools down it does not get cold.
Keep checking on the babies. See if Mom comes back. Wait approximately ten to twenty minutes of crying. It will feel like eons but Mom needs time to come back. If nothing bad has happened to her, she will return.
When she does come back, give her support. If Mom is friendly, consider bringing her inside to raise her babies in the safety of a home. You can also crate them in a garage.
If she isn’t friendly, or you can’t take them in, then put a nice big box out with towels for her to nest in. Make sure she has plenty of food and water. Because of instinct, she will move the babies at some point. The more inviting you make her stay the more likely she will continue to hang around. The kittens will start roaming at about 3 weeks old, and they will start eating solid food at about 4 to 5 weeks. That’s when you’ll see them show up for dinner with Mom.
Again, if you can’t do a full-fledged rescue, it’s okay to T/N/R (trap/neuter/return) this family when the kittens are old enough. Don’t feel guilty you haven’t been able to find them homes or bring them inside. The most important thing is that you don’t ignore them. This family needs you.
Families do well together. It’s okay to T/N/R the kittens, but remember cats are domestic animals. They rely on humans for food. A regular meal and sterilization do wonders for the quality of life of cats.