Kids are smart. S5 figured out very quickly that W is the WAS and that I'm pro-M. I think it is apparent thru action and attitude. He also has commented on how W is 'sad' and 'angry', and notes that I'm 'happy.' I imagine you're kids likely know your sitch as well.
Going against the politically correct pro-D movement, I decided to not fall on the sword so to speak, and take blame for having 'chosen' D alongside W. That is W's choice, and I'll let her take the heat for it. I just chose not to destroy all of us by battling her D.
I have told S5 that I have hurt Mommy's feelings in the past and I've made mistakes, and that I am doing my best to be a good man and to not hurt her anymore. At his level, we talked about how a person apologizes to the person they've hurt, how the other accepts the apology, and how they put it behind them and then move on. I think accepting 50% of the responsibility for an M's problems, but allowing the WAS to sit with the choice of D is just fine. S5 has commented how W doesn't apologize or admit fault or forget hurt - to him or me. I can't help her there - she's painting herself in his eyes.
IMHO, with D, kids may choose sides, blame one or another for abit, but as they grow, they'll see things more complexly. Especially if we don't badmouth the WAS. I stay very positive about her, and don't say a thing about MIL/FIL.
