You could try talking with them, reasoning. Kids aren't dumb, especially when it comes to manipulation
You did the right thing, don't make the mistake of letting them run amok because you don't wanna be harsh.Children grow up best with some actual discipline, doesn't mean you have to lock them in the cellar with regular beatings but if they are being little bastards then take their toys away and send them to their rooms.Sucks in the short term yeah, but a 5 year old being a bugger is a lot less trouble than a 13 year old being a bugger.
Your ex wife is not disciplining them at all so she can be the "good parent" in the divorce. Happens all the time. You should talk to her.