Here's a ROUGH plot I came up with, tweaking the original prompt from a tribe to one child and a millennia to a century. Hope you like it! (Please don't steal it; I kinda want to make it into a book)

Boy wakes up all alone in the middle of a vast desert with no recollection of how he got there. On his back is a backpack of full of supplies which he will need to survive. On his travels he comes across advanced abandoned machines, decaying in the hot desert environment. This piques his curiosity as he tries to find a story to the ancient - yet futuristic technology. He comes across a girl and they become travel partners. They continue, trying to find the reason behind this. The boy keeps getting flashbacks to before he was sent to the desert, he fell asleep in 2017 in his nice, middle class house, only to wake up in the middle of the desert in 2180, according to his watch. The boy and the girl keep being joined by more and more kids and eventually they form a community. The children just start to run out of resources when they see a forest on the horizon. They travel towards it, hoping there will be water, because they are almost out. When they reach the forest, they find it to be populated by older, dead trees, and young saplings. They replenish their water and food in the forest, and continue onward. After a month of travelling through mountains and more forest beyond, they come across some old missile silos, completely empty. They slowly piece together what happened using the silos, flashbacks, and pieces of evidence left by the people who put them there. What happened is that the planet had entered nuclear war, resulting in a nuclear winter that lasted decades, killing off all life on earth, except the children, who were placed in cryogenic storage. Tree seeds had survived, but that doesn’t explain the animals. Who had put the animals there? Who had put the children there, if everyone had died?