How do I stop my 18 month old dogs from eating their feces?

Your dogs should not be eating puppy food. Dogs lose their puppy teeth at about 5 months old. That was over a year ago for them. I second the advice about feeding a more digestible food. Two of the most easily digested foods are National and Blackwood. They are made at lower temperatures than most other foods. That makes them easier to digest. I also suggest making sure they get enough to eat. If they don't feel hungry they are less likely to eat crap.