I find when you are just starting them on Kongs, you want to make them easy. Stuff some kibble in and then "seal" the end with a little peanut butter, cream cheese, or the Kong stuffing they sell. Once they get the hang of unstuffing the Kongs you can start making them more challenging by freezing them.
Here are some frozen Kong recipes.
Equal amounts of dry dog food and canned pumpkin
Warm water
1 to 3 spoonfuls of canned dog food
Mix this up until it is very thick. Use a small spoon to stuff the mixture into the big end of a kong.
Put the stuffed kongs into a plastic container and stick them in the freezer. The next day, give your dog a meal in a kong or two.
Another frozen treat is homemade frosty paws
1/2 of the large plain yogurt containers
3 heaping spoons (teaspoon or tablespoon - doesn't really matter) of peanut butter, sugar free preferred.
2 bananas.
Blend…then pour the mixture into the small little 8 oz dixie cups - and freeze. Or mix with kibble and stuff into a Kong and freeze.