Hi L.,
my 4 year old sometimes has the same issues and what we've found to work the best is to make sure she drinks a lot of water and a bit less milk. We still make sure she has the milk consumption she needs but we gave her a water bottle (princess) to help her drink more water. We also started using Fletcher's children's laxative which tastes like Rootbeer and is very safe. Our pediatrician recommended it to us and the great thing about it is it tastes great and within an hour causes a bowel movement. We probably use it once a week just to make sure she stays regular but with the water consumption it really has helped.
Hope you find that useful.