How to get my boyfriend's kids to like me?

My bf and I have been dating since March but I haven't spent much time with his kids apart from the odd dinner together. I am now pregnant and will be moving in with him and his 3 kids and am looking for any tips or past experiences. They are aged 8-16. I'm worried that their mom may have told them some bad things about me and that my age may be a factor. I'm 22 (boyfriend is 37) and this makes me only 6 years older than his eldest son. Any help would be appreciated.