Tell your new Jr hires it is normal. It is normal for anyone to not know a brand new codebase. Even the most experienced programmer will need an acceptable amount of time before being acquainted with a codebase and architecture. They only know how to ask better questions.<p>So teach your new hires and guide them towards the right question.<p>It is not up to the new hires to get our of the imposter syndrome. It's most likely because you are making them feel that way, asking for the impossible.
I have come across the opposite problem: new hires who think they know better than the greybeards. There doesn't seem to be any correlation whether they have gone to college or self-taught.<p>What do you do without bruising their delicate egos?