An older children's/young teens book. Modern, not a classic from my childhood.
The narrator is a 12yo boy who looks like an adult. He goes on a trip with a friend, pretending to be her father. There are several other genuine parent-child pairs there, too.
The children go on a space rocket trip, and he ends up accompanying them as a 'responsible adult'. The organisation sending them on the rocket turn out to be devious murderers, I think, but he saves the day (and their lives, of course).
Rings any bells?