Finding kittens outdoors can be concerning but try not to move them unless they are obviously in danger. Just because they are alone does not mean Mom is not taking care of them. Mom cat often leaves young kittens alone for hours so she can forage for food.
If you find kittens try to observe them from a distance every couple hours for at least 12 hours. If they are content and not fussing more than likely their mom is coming back.
Here is some more information from our friends at Best Friends Animal Society!