@CreatingHavoc
Thank you all. This post has made me realise just how bad it all is. I can't believe I've become numb to it all. I feel awful.
Please don't beat yourself up
@CreatingHavoc these things creep in subtly.
If you were able to magic you from the start of your relationship into the midst of all this awfulness that has built up by stealth over the years and is happening now you would be horrified and run a mile.
Your DP will have been subtly eroding your boundaries and over time he will have normalised smaller unhealthy or unsavoury behaviours and then gradually escalated and expanded till he has included his sexual boundary crossing repertoire of both you and the children.
Please do seek help, speak to Women's Aid and I strongly encourage finding a qualified and accredited therapist.
You are not to blame for his shitty actions 
What's important now is that you take steps to protect yourself and your DC as soon as you are able.
I expect you have told him verbally and physically many times that you are uncomfortable with him touching you in a sexual manner in front of the children and he's not listened. So don't expect him to suddenly stop doing this, you are going to have to take active measures to keep safe I think.
Tell someone you trust in real life, a friend or family member. Don't be ashamed, you don't have anything to feel shame over so don't listen to that horrible little voice inside you that wants you to keep it secret from anyone 