Since I have never coached younger than 16 I am too asking myself what the difference will be. I always try to see my role as a coach on these levels:
Strategic: what are the players capable of (skills, etc.) how do I want them to play, what does it mean for the things I do more as a coach.
Tactical: in situations on the field, what kind of decisions to I want my team to make
Individual: what are the motor skills needed
Does this help? For each 'level' you can ask yourself a lot of questions. For you having experience with mini rugby the tactical and strategical might scare you: my advise would be:
develop a style of play where everybody is involved
focus on open play although working on set-play might be easier / more recognisable
have fun, play lots of little games
make the girls aware that there is now another dimension to playing rugby: relate training to playing, try to make the team "coachable".