Put their kibble down for 15 minutes if there is any food left it goes back in the bag. They don't get more food until dinner, same thing as the first meal, put it down for 15 minutes and take away whats left over. Don't watch them or add anything to the kibble. They will learn quickly that if they want food they better eat it when it is there. A healthy dog will not strave themselves! It may take a few days but they WILL catch on. My dogs inhale their food in about 30 seconds. When we got Toby he had been free fed. A few days of strict meal time and he gobbled all him food down.
Adding stuff to their food will just make the picky eaters, it is just a quick fix and doesn't help you in the long run.

Good Luck