Question for people with children or step children

My bf has a 9 year old daughter and though she knows her father and I are "roommates", she doesn't know we are in a romantic relationship. We have been in a relationship for a year now, and his daughter is quite comfortable with me. But again...she doesn't know we're dating. We don't kiss or say "I love you" to one another when she's with us (he gets her every other weekend and 3 evenings per week, she does not live with us full time).

Tonight, she and her dad (my bf) were getting ready to leave- he was taking her back to her mom's house- and out of habit, he leaned down to kiss me like he always does, but stopped himself, remembering that his daughter was in the room too.

My question is: when should we tell her we are in a relationship? Should we start showing affection to one another around her? Have any of you dealt with this type of situation? What did you do?