First thing you have to understand is that in a leadership role, tasks involved in being a leader take precedence over your individual tasks or you become a bottleneck to the whole group. You should have taken the time to help the intern get started, that is you highest priority unless your boss tells you differently. Let him know that you are doing that and that whatever you were working on will be delayed while you get this person started.
Since you have other new people coming in, you need to take at least a day to prep for them. It's discouraging to come to a new job all excited and then find no one has any time to give you anything to do. That makes for morale problems and people who leave very quickly. It is your job to prevent that by being prepared. Make sure they have the equipment/software they need (I had one job where I didn't even have a computer for two weeks). You don't have to install the software as long as you have the licenses and the locations of the software for them to do the installations themselves. You want to have a list of what they need to have and a policy for what other stuff they may want to install. What approvals do they need, etc. You will want to create an onboarding document with the source control directions and locations, an overview of the project(s), build processes, QA processes, unit testing requirements, coding standards etc. whatever you need to have, key HR policies.
As a company grows, you need to formalize things, so you and your boss should get together and decide what the policies need to be.
On their first day, you will want to get them started by installing the software and looking through the code repository to get a feel for how the project is progressing. Then you will want to talk to each person individually, find out what he/she considers to be their strongest skills and what the are most interested in. Then make assignments based on that.
For all new employees, institute a code review even if you do not currently do that at your workplace. Never let a new employee commit unreviewed code. The place to stop people who are going in a wrong direction is at the beginning. This is even more critical if your new employees are not experienced such as interns.
If you are doing agile, you likely have a daily standup meeting. If not, then at least do a weekly meeting to find out what progress they have made. One of the worst mistakes you can make is to find out that someone was making no progress for weeks without an intervention from you.
It helps to get them started by doing pair programming. Don't insist on being the one at the keyboard however, it helps them to learn if they do the actual typing even if you are telling them what to type. And try to lead them to conclusions rather than tell them outright by asking leading questions. Teach them to google for answers, to understand requirements and ask questions about them and translate them to code, to look at edge cases, to handle exceptions, to do unit testing, etc. Talk about the meaning of what you are trying to accomplish. Why are we doing this, help them develop judgment about selection of tools and techniques to solve various problems. All of these things are done through asking leading questions.
Cut your own workload back. You should plan at least half your time managing the workload of the new employees, providing training and responding to questions, doing code reviews, pair programming, etc. The faster they get up to speed, the faster the overall project will go, so you need to take the time to get them up-to-speed.
After they are doing well, you can cut back but at least a couple of hours a day will need to be done on leadership activities.
As far as getting your own work done, block out a time of day where you are generally unavailable unless it is an emergency. Make sure to give them an assignment before you disappear for four hours to code though. At first this can be readings, code documentation, etc. until you feel they are ready to code on their own.
And don't grab all the good assignments for yourself. It is a better use of your time to give them some challenging things that you guide them to success with than for you to do everything because they don't know how. If they don't know how, then teach them. If they make a mistake that you catch in code review, tell them what to fix, but make them fix it themselves or they will never learn. People tend to live up to the expectations put on them by their supervisor. Make sure you expect them to grow and improve.
Above all, allow time in the schedule for these few weeks of learning. Everyone will get faster as they learn, but not until they are up-to-speed. Don't put unfair expectations on them.
