Good Day,
When my kids had a problem with constipation, I did one or both of the following:
1. Give some applesauce in their oatmeal, or by itself.
2. Give some warm apple prune juice, from the baby section.
3. With the child lying on its back, gently push with your hand their feet into their stomach area, then alternate one leg at a time, and then the two together again.
I did #3 while playing with them, and they liked it. I also made sure that they had plenty of water. I was not one for a lot of laxatives, so the at home babysitter told me about the applesauce.
If they are drinking milk, you might want to change to 2% milk and see what happens. You can mix the two - whole and 2% and see how they like it.