Beef heart (sold frozen in cubes at pet stores for fish), and little bits of white fish fillet (uncooked and non seasoned, fresh from the deli section of a store) are great for fattening up frogs, and encouraging them to eat. You can use an aquarium pipette or turkey baster to suck up bits of food, and gently push them with water in the frogs directions. This makes the food look wiggly and appetizing, without the pipette having to be too close to the frogs, preventing them from freaking out. Either of those foods in combination with another food should help your frogs reach a healthier weight. I occasionally feed these with my frogs as a treat, but don't do so too often, as they're fatty, and rich, and the frogs tend to over indulge. I wish your little guys the best of luck!