I got it right but didn't bother to dredge up my algebra skills (which I'm fairly sure I didn't have when I was 8!). Try this instead:

Pile 1 has 4 more coins than pile 2, so it has to have at least 4 coins in it. That would make it 4+0+1... oops, can't have 2x0, so that's not the answer and doesn't use all the coins anyway.

Try again with 5 coins in pile 1. 5+1+2+2 works okay except they don't add up to 20.

(By this point she'll probably be happy enough to skip stage 3..., but obvs it's 6+2+3+4=15)

Then try it with 7 coins in pile 1 and it works AND adds up to 20, so you can stop.