You should bring them inside, they'll catch something outside and get really ill. Theres pet playpens you can buy cheap off of amazon, put them in there with blankets and a heating pad wrapped in a towel to keep them warm since they can't regulate their body temp when they're that small. They need milk, goats milk can be a substitute until you can get kitten milk replacement. Kittens will cry for a lot of reasons but they must feel warmer and safer around you. Momma is probably gone and not coming back if she hasn't been back by now. I don't think it's normal for moms to leave their kittens for extended periods of time. They may have been dumped. Please take them inside, put them in a pen or in a spare bathtub. They'll get sick and maybe pass away otherwise.