A toddlers diaper should be changed when wet enough which is usually every 2 hours but sometimes can be 3 hours between diaper changes.
You can feel the diaper and see how squishy it is and if it's really squishy then it's time to change the toddlers diaper.
If they poop the diaper then the diaper should be changed as soon as you notice they poop the diaper. And if your toddler poops the diaper you will certainly know because you'll smell it.