
Is it possible you've been giving them too much, too soon? I don't know what breed you have, but when I start to wean my dalmatian puppies I only give them a teaspoonful each once for the first two or three days, then a teaspoonful twice a day for another few days, and gradually increase the amounts. At this young age they still need mum's milk in vast quantities, with only a very little solid food.
If the pup has diarrhoea I'd call the vet for advice.