I would actually not feed milk until the scour cleared, only on Lectade or similar, you can make your own from water (about 1 ltr, boiled and cooled, can also be camomile tea) honey (2 tablespoons, but not sugar) and salt (1/2 teaspoon) and baking soda (1/4 teaspoon). Have got the quantities somewhere. It could be caused by too much (possibly damp) greenery too soon after milk. Especially if they are bottle fed.
But watch out for scour going black, that could be cocci.