Basic Recipe
1 cup whole milk Kefir
1/2 cup banana (approx. a small banana)
2 Tbs Nestle Nido
1/2 Tbs honey
Kefir is more sour than milk, which is why I've added honey. You could also add fruit juice, sugar or anything a little sweeter.
The best part of this recipe is that you can really substitute anything. If you can't find kefir, you can use equal parts milk and yogurt. You could also just use milk or half and half. If your child has a milk allergy you could use coconut milk kefir or any milk substitute that you like.
If your child doesn't like bananas you can use any fruit, you could also forgo the fruit and add a splash of vanilla or chocolate if your child likes that. Again you can also use formula in place of the nido, which would make it a more balanced option if you're using it as a meal replacement.
You'll notice in the nutritional information below that the kefir options appear to have more sugar, but it's unrefined sugar - a mixture of fruit, honey and milk sugars. You'll also notice that there isn't vitamin information for the kefir options, it's because they are listed as adult servings on the package and I didn't want to do the math.