If the child's or toddlers diaper is dry then you can usually leave the diaper on if the diaper is a disposable diaper that is clean and hasn't been used.
If the disposable diaper has been peed or pooped in then you need to remove the diaper because the diaper being wet or having poop in the diaper can add extra weight to your child's weight reading and make it inaccurate.
Most doctors do recommend removing your babies or child's diapers and weighing them naked and some doctors offices that deal with children and babies have spare diapers in the doctors office that you can change the child or toddler back into a new diaper.
If you do weigh the child or toddler with the diaper then you need to know how much the wet diaper or dry diaper weighs and then subtract that weight from the child's weight reading.
When I took my babies and toddlers to the doctor and weighing them when they were in diapers I would remove the diapers and then put it back on if it wasn't too wet or change them into a fresh diaper after weighing them.