Meeting children

So I'm meeting my BF's children for the first time. 4 & 3. We are going to the park next weekend. I'm not sure why I'm nervous because I've wanted to meet them, but it's a tiny bit of pressure. I keep thinking what if they don't like me? What if they don't want to open up ? I dated a guy before and he left me very shortly after I met his child. His child didn't open up or talk to me at all despite my efforts. He just said "No" to everything.. Even questions that didn't require a yes or no. That day my ex grew a bit standoffish and more and more distant as the week went on. Somehow I fear it will happen again. I know children can determine for some parents who is "the one." && I remember he mentioned when we first dated that one of the things he wanted in a woman was that she be the "right fit" for him and his children. They are his pride and joy so naturally I want them to be a part of my world as he is. So I'm just nervous. Any pointers for meeting them. I know this will be a new experience for them.