What should I do. Please help me.

I suggest taking them to the vet. They could have a virus, or parasites.

Worms! Get a sample of their stool and take it to the vets for analysis.

My dog had this too. Put him on prescription dog food- low residue formula

This could also be caused by feeding a dog food with corn and soy in it. These are commonly used in box store foods as fillers. There's no nutritional value to either of these products so it's passed and you end up picking it up in the yard. If you've had them tested and there's nothing wrong with them, you might want to try a food that is corn and soy free. Most find that by doing this, they feed less and pick up less.

Let us know what happens.