The Dark Side of Nowhere by Neal Shusterman, published 1996. There are a lot of similarities to the scenario you're describing. I thought of the story immediately, but had to find the book name and author before writing it as a reply.
The alien scout-ship crashed, disguised as a tornado. Between that and the radiation, they killed about a quarter of the town.
They take on the identities of some of those who are killed, using their dna, and used the excuse of an epidemic for the rest.
Their children are similarly hidden very young and brought up not knowing their history. This changes when one of the adults (who had his satellite pointing the wrong way listening for aliens) hears the signal that the fleet is finally coming, and he works to recruit the kids.
The kids (not the adults) have to take monthly shots to keep looking human (because their bodies reject the alien dna as they grow), it fails as the kids go through puberty and they won't be able to replace the disguise because the original dna is lost.
The main character looked exactly like a dead man, one JJ Bridges (having been hidden with his dna), whose elderly mother is found at one point, and recognizes the similarity. I think he might have fought in vietnam, but he didn't die there (or they wouldn't have his dna).
The lead (and a few others) ultimately decide that the invasion shouldn't succeed, that this conquest is wrong, they have lived as, and so become, more human than alien. They use work to halt the adults' plan (which was to sabotage human society so they'd be easy pickings) and warn the government about the coming fleet. The lead's parents also use sabotage, having come to the same conclusion earlier, but believing the kids weren't listening.