You can raise this need by simply going using the toilet. Nothing can affect this need. You can not raise your Sim babies' bladder bar, you have to wait for the baby to wet them selves.
For adults if you don't put them on the toilet they wet themselves and it takes 3 seconds to clean up.
|Action||1-Star Object||2-Star object||3-Star Object|
|Use toilet||7 secs||6 secs||5 secs|