Put signs up: LOST CAT/PHONE. Your kitties might have gone by someone's house, and maybe they took them in. If they see the signs, they might call you.
GO TO THE SHELTERS. If someone found them and took them to the shelter, the kitties might be there, just waiting for you to get them. Don't call the shelter because they are terrible at describing animals there, and then you might miss them. You HAVE to go there, each couple days.
If you get them back, bring them in at night, and only let then outside during the day. I hope you find your kittens.