I know the traditional plaster would be too strong for a child, but I did find a "recipe" that used less mustard, plus "soda" (baking soda?), and eggwhite (even for adult plasters, I've read that if you use eggwhite instead of water, it will not blister the skin). Ds has a horrible congested cough and I'd love to get it to break up more for him - and I know of someone who used a plaster (I'm thinking it was the traditional kind!) on her ds (same age as mine) who has asthma & had a bad cold and she was very impressed at how well it worked.
Just hoping to get your thoughts on it. Anyone ever try it? (This is for putting between layers of fabric, btw - not for directly on the skin!)