
Hi Lisa, what breed are they? How often are you feeding the puppies, and how much? It won't be their mother's milk making them runny, it's far more likely to be the solid food, especially if they are eating too much of it. Lots of small meals is far better for them than a couple of large ones. They only have very small stomachs, which are not capable of containing all the food they need for growth, in just one or two feeds. Overloading their stomachs, if that is what is happening, will certainly give them the runs.
PS - also, how often have Mum and pups all been wormed?