bignbuttons this must be such a stressful situation to be in.
I have never had to deal with SS, but as an outside observer I believe it can only be good to be honest with SS about the information your ex misled them on, just stating the reality of what you know and do and feel, without necessarily castigating him for misleading them (rising above the blame game he's playing).
Eg.: "The DC are well fed and cared for, here's their schedule, here's what I cook for them, etc. When DD1 ran away I was unaware of it because she regularly plays in the garden or in x location with the village children, and she knows to come home in situations x, y or z. Her action that day was out of character, and I spoke to her about it afterward and she was able to raise concerns about a, b and c.
My H and I are splitting up at the moment, which may be adding to stress for the children. I have concerns about his treatment of me and the effect it may be having on the children, which is why I have chosen to split.
For example, earlier this summer he said x to me in y situation. This is similar to another event last year where he did x when y upset him, and then blamed me for his own actions. Other incidents include ..[state the facts]..
I am often scared and confused by his behaviour, and concerned about the effect it may be having on the children. So after x amount of time trying to work things out I have concluded that the situation is unlikely to improve, and that a separation/divorce will be healthier for all of us.
etc etc."
The facts are in your favour. State them clearly, and know ahead of time the important points that you want to put forward. If you're not asked directly about them, don't hesitate to volunteer them, ie: "I'd like to make clear that ..[all the stuff above].."