If you choose, and this is suggesting compassion from you that I think might be unreasonable to expect, you may talk about your ex as someone who has had major problems handling his finances and economically contributing to the family. Say that the law had no choice but to let him know how serious his problem was; they had to arrest him because he was showing he couldn't help himself. Explain that you are hoping he'll get the help he needs to be the responsible person you know he wants to be down deep.
Tell your ex how you have handled explaining his arrest to your kids and also tell him that your explanation can't continue if his behavior doesn't change. I know you could be seen as the villain here, because if you didn't report the non-payment the cops would not know about it. Don't own that villain's role. If he puts you in it with your kids, that signals a shift in how you can represent him to his children. If he truly had concern for his kids, he would reassure them this will never happen again, live up to his financial responsibilities and resume a good relationship with his kids.