When your toddlers diaper is leaking it is usually caused by the diaper being too small or too big and not fitting tight enough around the toddlers body.
For the diaper to not leak the diaper needs to seal around the legs especially when the toddler is standing and if the diaper is sagging and loose around the legs then the diaper can leak poop or pee out of the leg gathers.
If the toddler is sleeping and laying down then you should make sure if it's a boy that you point his penis downwards so the pee goes into the diaper and not up the waistband or out the side of the diaper.
Make sure the diaper fits tight enough but not too tight.
Also you may need to cut a slit in the first diaper and then put another diaper over the first diaper to make the diaper more absorbent for heavy wetting toddlers.
You can also buy diaper booster pads that you can put inside your toddlers diaper to make them absorb more through the night or when traveling.
Another option is to buy better brand diapers such as Luvs diapers, gentle steps diapers, Huggies diapers, Pampers diapers etc.
Or use some plastic pants over the diaper to catch any leaks.