My 3yrs old girl go to preschool. And she seldom drink water so cause her constipation. Any solution ?

my boys do having this problem and every time he have to go to the toilet end up crying cos due to constipation. and cannot poo out . i have try lot of fruit like banana payaya prune and also did let him have more water bit still the same. end up i try yogurt which really work for him. maybe u can try too. and also the longer days she never go poo meaning their poo is getting more and more hard inside. try if she can poo daily. this also help too