I agree positive parenting classes would be good. Since they are 6 and 8 - have them sit down with you with a pencil and paper. Ask for their input on how to get along better. It won't be a pointing the finger blame game but a solution game. Have them write down rules they have to follow, when they have a conflict like - no name calling and not hitting each other. Teach them good problem solving like knowing when to walk away.

Try once a day to play with them something like a board game or video game. Often children fight when they're bored. Since you mentioned marital problems, perhaps they need to talk about their feelings. Good luck