How do you tell them? Write down what you want to say. Read it, edit it, re-write it till you feel confident about it. Put yourself in the position of receiving that message and if it sounds good, tell them exactly like that.
If they can't understand you having another job, they're out of their minds. Who can live off of a few hours of work here and there? So no matter how you tell them, it's not unreasonable that you'd have another job. Ya know?
Also, congratulations again on that new job. I hope you like it. Grace of Jesus to you, brother!