You're right, if you receive any kind of money for training horses or people you lose your amateur status and are considered a professional. This really only affects you if you show.
I always start off my lessons as privates until they can post and steer on their own, then I move them into groups. Depending how old they are (usually the younger they are the longer it takes) you'd be surprised at how long it can take to get to that point!
Patterns are fantastic: cutting across, circles (surprisingly tricky for some kids to learn!), serpentines, reversing, etc. I put them through easy patterns and call it "Dressage day". Haha :) I put them on a lunge line, tie their reins, and make them do different things with their hands: on their head, airplane arms, sing head shoulder knees and toes... :) Lots of stuff with ground poles to lead to jumping: make a 'jumper course' of poles and work on finding the straight line down the middle of the pole. A fun game is to put 2 poles out like a line of jumps (like 48 feet apart) and get them up in their 2 point. Over the first pole they put one hand behind their back, and over the second one they switch hands. Then when they get balanced enough to do that I see who can switch their hands the most inbetween the 2 poles. When they're steady enough I let them ride without stirrups a little (again, starting on a lunge line is great!). In fact, my students aren't allowed to canter until they can do a solid posting trot without stirrups. Games (which people have alreaday mentioned) are fun, especially when you incorporate horsie vocab like inside/outside (simon says put your inside hand on your outside knee, etc).
I'm excited for you! I love my job! :)
PS- I'd look into getting a good liability form if you don't have one already!