No you do not need to use baby powder when changing a babies or toddlers diaper.
Baby powder can make the baby smell better but the baby powder is not needed and actually for girl babies the baby powder could cause Uterine Cancer later on in life.
So I would avoid using baby powder on baby girls and even boys do not really need baby powder when changing their diaper.
The baby powder really just makes the diaper change harder because it creates a mess on the babies bottom and groin and you need to wipe the baby powder off the baby and that can get messy when the diaper is wet or even when the baby powder comes into contact with poop.
I only used baby powder on my first kid when he was a baby and after having a mess to clean up with the baby powder I stopped using it.
As long as you keep your babies diaper changed often enough and wipe them up and change poop diapers right away or when you can as soon as you notice it then your baby or toddler should not get a diaper rash.