The best diapers for toddlers that don't sag are the Pampers Cruisers diapers or Pampers Baby Dry diapers.
Both pampers cruisers and pampers baby dry diapers have extra absorb channels that prevent the diapers from sagging as much as other diapers.
Also Luvs diapers don't sag as much as other diapers either.
Although the diapers do still swell up and get puffy when the diaper is wet.
A good tight enough fitting diaper is also key to preventing a saggy diaper as a too loose diaper can sag more than a diaper that is as tight as it should be.
If your toddler is wearing a diaper that is too large for them then the sagging can also occur.
All diapers will sag some though because they get full of pee and when it gets full the diaper should be changed as soon as you can change it.
Gentle Steps diapers sag some but not as bad as other cheaper diapers.
You can also put a Onesie on your toddler to help hold the diaper up when it gets wet.
Personally I like seeing my toddlers saggy diaper in their pants as they walk around as it's just too cute and it's cute how the saggy diaper makes them waddle some.