You can always wrap the powder in a slice of meat whether it be lunch meat or raw meat and give it to them. Make the piece with the powder small and easy enough just to gulp down. Have multiple wraps or pieces of food ready so you would be feeding really quickly one right after the other.
That would definitely work for the girls because they just hoover food into their bodies.
As with most things for Angus, he eats very slowly and most of what he eats at some point falls out of his mouth. In the past, for various things, I have tried peanut butter, honey, cheese slices (I wrap whatever up in the cheese, pinch the sides to close it and it looks like a little ravioli almost), lunch meat, putting things inside of whatever he is eating (could naturally have a cavity, or I try hollowing out a space), hot dogs etc...
None of those things have worked because he eats so slowly that the moment he realizes what's inside, he spits it out and looks at me like I have betrayed him. The cheese thing was for some antibiotics he was on and it got to the point where he would unwrap the cheese in his mouth, spit out the pill and finish the cheese.
You should see the way he eats hot dogs when I use them for training. He will take the little hot dog sliver in his mouth, nibble off the "meaty" part and spit out the casing portion. It is absolutely ridiculous and I really should try to get a video of it. Apples are the same thing, he always spits out the skin.