What if you tried putting the Epsom Salt in capsules? Or maybe you could try a small ammount of lobelia for the same relaxing effect on the bile ducts? A small ammount though-- Too much lobelia will make a person throw up. About 1 tsp will make an adult throw up.
I don't have personal experience with children doing flushes. These are just ideas.
Good luck. Whatever you try I hope you will let us know what happens.
Gardeninginthefog