The bladder is one of the six needs your Sims have on the Happiness bar. It is represented by a toilet. There is no way to lower this need it will gradually lower by itself. You can raise this need by simply going to the restroom and using the toilet. Nothing can affect this need. You can not raise babies bladder bar you just have to wait for the baby to wet them selves.

For adults if you don't put them on the toliet 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