I'd take the food to the frog not the frog to the food. Use a turkey baster to shoot the food at the frogs gently.. just make sure it's a new turkey baster and not one that's had soap used on it as that would be toxic. I know african clawed frogs can go a while without food so I assume african dwarf frogs also do not need to eat every day.. I would give them a few days to get used to their new surroundings they'll eat eventually. Don't keep wasting food since it may cause water problems.. remove any uneaten food.