Jon's stuff is good
As for the willow being toxic, I can only recount that I spent an entire Summer demonstrating and making willow bast cordage. I can tell you that goat willow is so called because it tastes sweeter than any of the others, and I reckon the goat's prefer it.
I do know that I didn't have a single ache that Summer either
so I guess there was just enough salicylic stuff in the bark to work wonders (yes, I did use my teeth to trim off ends, etc., I was working in the bronze, v. early iron age)
I reckon that Robson Valley has it right; unless someone's actually allergic to aspirin, then I really don't think there'd be a problem.
Elder, Rowan, Willow… we used them all as kids, and while it doesn't seem to have done any of us any harm, modern kids don't get grubbily filthy outside playing day after day after day the way we did when growing up…so, hmmm, I'd do it, but I don't know whether some H&S requirements mean that you should or shouldn't.
Sorry, not being much help
M