This comes under functional constipation usual in this age group of children. Causes anal fissure and painful defecation. It creates a vicious cycle of hard stool causing pain and pain causing suppression of urge to pass stool causing stool to become hard.
Next Steps
Always give solid food first thing in the morning not milk.
Health Tips
Always make them sit on potty after feeding for 5 minutes causing stool to pass effortlessly.