Perhaps you deal with it by building it into the schedule: Set aside a specific time (or times) during the day where junior members have stand-up meetings with senior members.

Hopefully getting the interruption onto the schedule would have the benefit of allowing everyone to plan for it and try to minimize its cost. And it might also encourage junior members to try and work through problems themselves first (because they have to save questions for scheduled question times), but also make them feel less like an imposition when they do seek advice.