There are hundreds of different types of breathing exercises, so I will mention a few and suggest important concepts to cover.
1.) Teach them how to visualize their breathing. This one is extremely important and is the root of good breathing. I always teach my students to think of taking big, large, open, relaxed, deep breaths.
2.) They should be silent. If you hear them breathing, there is restriction and therefore inefficient breathing. They should be silent like ninjas.
3.) They should be relatively quick. A deep, full breath does not take a long time to take - less than a second or half-second with some training. Quick breaths like the latter mentioned are called catch breaths.
4.) Lungs should be completely full. They shouldn't be able to take in any more air after taking a big, full breath. If they can, then they need to try and inhale more air the next time they go to take a big breath.
5.) Breathing should be natural. They shouldn't take a big, wonderful breath and then hold it for 3 seconds waiting to come in at their entrance. Their breath before their entrance should lead to a natural exhale on the following downbeat (or whatever their entrance is.)
- Make sure that you have them also buzz on their mouthpieces while doing some of these exercises. It is important for them to feel the air move through their lips without any resistance. It is also a great way to warm up the lips before playing - something crucial for everyone, but especially youngsters with little or no technique.
1.) For visualizing, you can teach them the Four Corners exercise where as they inhale they fill their lungs down, forward, backward, and out (from your sides.) I demonstrate this motion with my hands and invite them to do the same. Here you're forming kinesthetic connections along with cognitive ones.
3.) For quick breaths / breath management, have them inhale for four beats and then exhale for four beats. Gradually shorten this to three and three, then two and two, etc etc. Depending on tempo / skill level you could also have them breath 8th notes. Alternatively, you can lengthen the exercise to 16 beats or even 32 to teach them minute muscle control and management.
4.) For filling lungs, have them take the biggest breath they can, and then have them see if they can add to that breath by taking additional tiny sips. You will find that all them will be able to take several extra sips or more. This will develop lung flexibility and capacity.
5.) For natural breathing, do an exercise similar to number three, but emphasis as smooth a change as possible between inhale / exhale. For different physical gestures you can have them bring their arms above their head so that their lungs are completely full when both arms are perpendicular to the floor, halfway empty when arms are parallel to floor, etc. This is another great way for them to monitor and manage their breath activity.
The exercises and concepts above are good for all age groups, however (and as I'm sure you've experienced) they must be presented in a way that is accessible to them - especially if it's fun. Have them pretend to be ninjas for their silent breaths, have them pretend to be monks for their natural breaths, have them pretend to be giant clocks when the bring their arms above their heads. What's most important is that their engaged while they do it, and for some people, making these gestures can be a little uncomfortable - especially when they are pre-pubescent and are socially hyper-aware.
Hope that helps.