My daughter is 12. While I have desperately wanted to take her for a ride, I have not felt she yet understands about hanging on, so without a backrest I won't take her on my Busa. I have ridden her around the block on my Harley but that's the extent of it (she loves it). My wife also rides, but she's our only child and neither of us could bear if she got hurt. About the ONLY reason I still have my harley is for those brief rides around the block with my daughter. In the next year or so, I'm going to gear her up and start venturing out further.
It is very adult of you to consider the wishes of the child's real father as you are the stepdad and he may object. While that's not a conversation YOU need to have with him, it is one that the child's mother needs to have, and you should respect his wishes regardless of her relationship with him; If he's involved in his child's life, then you should respect his wishes, as you'd want the same if the situation was reversed.