You are lucky that the boys are so young. I wish I had tips for what to say to your 6-year old, although I am sure many here will have great suggestions.
From a legal standpoint, just be aware that it might be difficult for you to unilaterally decide he cannot see his children. Legally speaking, parents who are married to each other without some sort of custody order/agreement in place have equal rights to the children. If he wants to make a stink about seeing the boys, absent some sort of protective order, or temporary visitation order from the judge, he will have the ability to make a big, big stink. And trying to withhold visitation absent a court order can look bad in later court proceedings for divorce/child custody. If you haven't spoken to an attorney yet (they will often consult with you for an hour for free), you might want to do so. Hopefully your AH will just keep his distance for everyone's well-being (and in light of his recent health issues), and give everyone time to settle down after all of the awful-ness.