I would say that teaching children about a healthy diet is a great first step. But on the same note, some of the foods that are really healthy (whole grains, deep green vegetables) are gassy foods. 

As for gas sneaking out during practice. I (late 20s) take an adult (mom-grandma ages) yoga class and sometimes during those stretches gas sneaks out. I personally have excellent control over that aspect of my body, but sometimes when you're doing an upside down reverse inverted cat dog pose, there is nowhere in your intestine for that gas to go but out. 

How we handle it as adults is that we ignore it. Because, hey, it's natural and everyone does it, especially during exercises. So while kids are likely to giggle, that's not a bad reaction.

What is a bad reaction is trying to single out someone who did it. Because now they're guilty of something bad and everyone's pointing fingers at them. This is embarrassing to a little girl and could lead to bullying. 

I would suggest you try to ignore it. If it's super obvious who did it, ask them to say excuse me and move on. This is the adult way to handle it. And if it's a constant issue, go over healthy diets again and put a glade plugin in the nearest outlet before class starts.