Explaining daddy is in prison?

My estranged husband was just sentenced to 15 mos in jail for domestic violence and violating protective orders. I have had sole legal custody since left him 1 year ago today. Our almost 4 year old and 17 month old daughters have had supervised visits with him for the last few months 5 hours a week and before that for 13 weeks 1 hr a week. What am I supposed to tell them? I would rather not lie about it but also don't know if they'll understand what prison is and why he has to go. Any advice would be much appreciated!