Getting your adoptive child to call you mom

Hello, I am in a current adoption placement that will be finalized in Feb. or March of next year. One sibling is 5 and the other is 10. My question is how do I go about getting them to call me mom? They were calling their foster mom "mom" before they came to live with me. When we first met I was introduced to them as Ms. (by my first name). That was fine with me because I have a long last name and figured they couldn't pronounce it. But now I no longer want them calling me by my first name. I told them I want them to call me mom now because I am going to be adopting them and they will have my last name. I also told them that they will be living with me until they are grown and I am going to be their mom, so that might as well get use to calling me mom now. I'm afraid that if they keep calling me by my first name, they will get use to it and won't call me mom. Of course if the younger one hears the older one do it, then she will too. They have been in my home for 3 weeks now, and we have been visiting and knowing each other for 3 mos. now. They do understand they will have my last name and they were ready for the adoption placement. They didn't have any issues leaving the former foster mom because she prepared them for adoption. They now refer to the former foster mom as Ms. instead of mom, but don't seem to like the idea of calling me mom. I have read some of the foster/adoptive parent forums and someone had said it has been over a year and their child doesn't call them mom or dad yet, which makes them not feel like a family. I can understand that. I have told them I didn't want them to call me Ms. (by my first name) anymore because it sounds like they are talking to their teacher or a babysitter and I am not either one. I explained to them the reason they were calling me that in the beginning was because we had just met. The 10 yr. old is withdrawn and is slow opening up, and has attachment issues, she also has a lot of influence over the younger one. Of course the younger one does and says what she does. Do you have any suggestions of what I could say or do to get them to start calling me mom? Also any suggestions on how to keep the younger one from doing and saying what the 10 yr. old does? Any advice will be helpful.