i am in a relationship and my partner has a 10 year old daughter from her previous marriage. i also have a 6 year old girl who lives with my ex-wife. we are planning to get married, but not sure how to break it to the kids. any suggestions?

I think you guys should plan an outing, a picnic of sorts with the kids being around and you two being there. All four of you spend time together and instead of divulging things let all of you get to know each other, and have these outings or meetings or dinners together more often so that kids are comfortable with the new people in their lives. I think it is all about making the kids comfortable and forming a bond with them. More you work on forming that bond, easy it will be for both of you.