
Usually if they have had a surplus of milk from mum the poo becomes rather sloppy, they should not dehydrate so long as they are feeding from mum, milk is a complete food, (which means it contains water as well). You can tell if a puppy is dehydrated when you pick the skin up between finger and thumb when you let go it should spring back. If they are round, fat, contented all is well usually, keep an eye on them.
If you are worried about them being dehydrated you can always give them cooled boiled water, but if mum is a good mum all should be okay.