As Skippy said - do they blame you for the split? When you choose a partner who's had a previous relationship you have to be aware that you're also taking on that relationship whether it's his children, if he wants to remain friends with friends of his ex (or his ex) or the ex in-laws etc. It's part and parcel of his life and all you can do is accept it. If you make it difficult for him to continue to see his children by causing ill feeling every time he goes to visit you're going to be the one who will ultimately suffer as you certainly can't ask him not to see them.
To me it sounds as though you may be jealous of the time he spends with them but you do have to find a way to deal with it. Children are very perceptive - are they being a problem when they visit your home because they know you don't want them there? Are they cold towards your OH because of your attitude towards them (or perhaps it's just teenagers being teenagers)?

I think that counselling may be a good idea as if you don't deal with your feelings it's going to damage your relationship (hopefully it hasn't got to that stage already).