as the queen of finding healthy fat sources to supplement my diet (i'm
UNDERweight), i have listed a few that have helped me:

first of all, i HATED soy milk the first time i tried it, too.  try
doctoring it up with non-dairy cocoa mix, make fruit smoothies, etc.  add
honey or sugar to sweeten - improvise!!!

if the child likes pasta, try mixing a spoonful of extra virgin olive oil
through the pasta after it is cooked, when you add the sauce.  i think that
this is one of the healthiest forms of oil that you can use/cook with.

lots of nuts, if she is not allergic.  peanut butter!  any kind of nut butter.

avocados, olives, yes, i loved them as a child.  what i do is make a
sandwich of mashed up avocado and tomato, mixed with a little honey mustard
dressing on a bed of lettuce (i add sliced artichoke hearts for a serious
'fat' sandwich).

hope this helps.

zenzele