Unfortunately-
2-door coupes and pickups are not very child friendly. Most are small to begin with, and it really can make it much harder to install a child seat and get a child in it without doors in back.
While it is possible that the lack of rear doors could make emergency access more difficult, the alternative is worse. Putting a child 12 or under in the front seat significantly raises their risk of fatal injury, even if the car does not have airbags.
In some small coupes and pickups, there may not be a good alternative other than selecting another vehicle. Usually, you can find a restraint of one brand or another that does fit in the back seat properly.