@Tamworth123 I'm sorry but I don't actually think your last 2 posts are particularly helpful. I honestly think they are just going to add to the fear and stress the OP is already dealing with.
Yes unwanted sexual touching of your adult partner when the DC are present is absolutely child abuse, but I think you are trying to imply that he might sexually abuse the children? There is no sign of this in the OP's posts that I can see.
I've given some other possible reasons upthread why a man would display the behaviours the OP's partner has, based on what she has said, it is clear to me that abusing her and violating her boundaries is something he gets off on.
FWIW I actually don't think it has anything to do with the DC themselves for him, I think the fact that their presence adds to the OPs discomfort and humiliation is what is doing it for him IMO. The DC are just pawns in his twisted game of control over the OP.
So far the OP hasn't said anything that would suggest to me that her DP is an immediate danger to the DC themselves unless he persists in wanking in places they might accidentally find him. But again I actually think it's more to do with the fear and discomfort it's causing the OP knowing this risky behaviour has happened and might again than it is that he is actually hoping to be caught by the DC IYSWIM?
His behaviour towards the OP in font of them is still child abuse however and obviously I wouldn't trust him not to wank in places the DC could find him which would be incredibly damaging as PP can attest.
@CreatingHavoc I hope you are ok and have managed to talk to someone IRL or had a chance to call women's aid 
I'd like to add to my earlier posts that you shouldn't tell him you are leaving him or that you are seeking help via Women's aid, these are flash points for abuse where things can escalate dangerously. Please stay safe.