Minecraft Mod Programming for kids and pricing/rates...

I've been teaching 8-10 year old kids Minecraft Module Programming for 1 1/2 hours each session twice a week from the inception of the 2015 elementary school year up until last month. I taught these classes for free since teaching was also new to me hence, teaching was a learning curve and the kids were in it to learn and have fun for free Tuesdays and Thursdays after school. Admittingly, my own awesome 9yr was my teacher with regards to learning Minecraft itself during and before lifting off with teaching mod programming. The class size was 8 and I plan to expand it to 12 students. I paired the kids up so that they would assist one another and it worked really well. Overall, the Minecraft mod programming appeared to be a successful 4 months per parent feedback. Now for the anticipating question...
Does anyone have any ideas on how I can structure a pay for learning model since my "free" trial basis appeared to work well? Hourly, per month, weekly, etc...? This is evidently a bit different from Managed Services agreement fees/rates which I understand much better.