This is a great idea. I've run a Junior club in the past, and you are going to deal with the age group that has possibly the most difficult relation to the hobby, because it's a time when girls (or boys), hanging out, cars or motorbikes, and a few more things become important.
From my experience, I'd advise you to be patient and open-minded. It's a good thing you're not restricting yourself to a particular type of modeling. Kids of that age, particularly the early teens, will also be very interested in wargaming stuff, so keep that option open too. Painting figures, even to 'play' with at a later stage, is also a form of this hobby of ours.
I ran weekly meetings, on Saturdays, late afternoon. Two hours was the minimum time because some of that time was required to set up the tables and materials, and to clean up afterwards (we met in a library, after opening hours). That left us about 1hr and a half of real modeling time each week. I ran weekly meetings because kids have a short attention span and do not want to wait another month to complete their model. I had a handful of experienced, adult modelers to help supervise the sessions and help the kids with the most basic problems to the most complicated procedures such as dioramas, airbrush and the like.
We often organised little, then much bigger shows and that was a great way to get everyone, whether they were model-mad or just in the club because a friend was there too, to 'be part of it' and 'own' the club. I had kids of 6 to 16 running and judging a competition, selling raffle tickets, running the (soft-drink only) bar, designing the artwork for the posters, supervising, getting the entry fee from the public and so on. We had a whale of a time. Our first show was at the library, basically for the locals and parents, but our last was at a prestigious venue, with sponsors, and a competition and exhibition bringing over clubs from all over the UK, France and Belgium.