Go the old BB route.....lower/moderate carbs off/cardio days....a crap load of carbs pwo....train weights 3x a week with workouts focused on strength and follow up with glycogen depletion/hypertrophy training. The build up of muscle will help to burn the fat on the off/cardio lower carb days.
Of course you might have to force feed some of them, as eating many calories is obviously something they are not used to doing.
If you go the whole milk route my money is on them getting bigger (with more fat).
This is a solid program to start with....just need to adjust calorie loads:
I'm guessing you must not be training in the US.....we have the opposite problem here!