Welcome to YT!
I used pennies in a can as you mentioned, and I found it extremely effective. I did it maybe a dozen times and Thor's barking went from a 9 to a 1. Now I am teaching him to Speak, and he's beginning to exercise his vocal cords again.
Also, Thor gets extra barky if he doesn't get enough exercise. Whatever you are doing, I would add 30 minutes of fetch or something else very tiring and see if that makes a difference.
It does sound like your middle child may believe she runs the household, or at least, she is resource guarding the people. There are basic things you can do to correct this. The easiest is to make her Sit and Stay before meal times. If she doesn't obey, take the food away and try again in a few minutes.
Please do not hit your dog, even a light spank is a bad idea. Modern training theory confirms that this is not an effective method and can actually cause behavior problems.
Finally: are your dogs spayed? Female dogs can be very territorial, especially if not altered.
Hope this helps.