A toddler or baby will be okay sleeping in the soaking wet diaper overnight but if you do notice the diaper is soaking wet during the night then it's a good idea to go ahead and change it to keep your toddler or baby comfortable and to prevent a diaper rash.
During the day you should change the diaper as soon as you notice it's soaked and don't leave your toddler in a soaking wet diaper longer than necessary as it can lead to a severe diaper rash.
Ideally you should change the baby or toddlers diaper before it is actually soaked and full of urine.
Check the toddlers diaper ever 1 to 2 hours to see if it needs changed and always change it once it gets nearly full or before.