How harmful is using pedialyte instead of apple juice for constipation in 1 yr old?

Not harmful at all, pedialyte is a balanced electrolyte solution, no harm drinking it. Apple juice is just fructose, a few vitamins, and water

It won't hurt, but is less likely to help. Apple juice has sugars that are poorly absorbed in young kids and acts as a natural laxative. Pedialyte is just modified IV fluid that is a good replacement for fluid losses.

Fluids and avoiding dehydration is one approach. Small amounts of prune juice may help. A daily probiotic is also often indicated. Please follow up with your physician. Pedialyte is more useful in rehydration. Adding age appropriate beans and grains is part of healthy diet.