Anyone can have the so-called "leaky gut" issue you mentioned. Thin intestinal walls is how a person is born. How long it takes to show itself as a problem is a function of the types and quantities of foods eaten.
I understand how it's difficult for kids to "be good" when they're around their friends and want to be "the same." I think as long as she stays with a healthy Diet for the majority of time, she is doing the right thing.
She, like everyone else, will one day need to discover what her trigger food(s) is/are. Nightshade plants don't bother everyone with psoriosis nor do citrus plants, but these are commonly listed as triggers. Proteins seem to be more common triggers for people. She probably doesn't need cleansing so much as she needs to discover what her triggers are.
About herbs: here's the deal. We've all consumed them since shortly after birth. It you've every had spaghetti sauce, pizza, alfredo, pot roast, and so on, chances are you've eaten quantities of oregano, basil, rosemary, thyme, and so on. These are all herbs.
Typically herbs has a synergistic effect meaning that nature has had lots of time to fine-tune them making their internal components work together w/o side-effects. Obviously common sense is in order with herbs as with anything else, but children will typically tolerate safe herbs as well as any other human person. It's just the quantities which need to be adjusted for their size. (By safe herbs, I'm talking about the generally accepted, non-toxic herbs used for centuries.)