You cannot 'train' for night dryness, it is a developmental thing where a hormone is produced in the brain to control urine output during sleep. This is very variable from child to child, and is so normal that health professionals will not treat it as a problem until they are 7yo. You can get advice with managing it from the school nurse or health visitor.
In the meantime, you can help by
not giving fizzy drinks, or caffeinated drinks like cola and hot choc, or blackcurrant drink after teatime, as these can stimulate the production of urine, as can milk in some children,
not giving lots of water just before bedtime and throughout the night, make sure the child drinks well through the day time, limit drinks between tea and bedtime,
if there is a long time between going to bed and going to sleep the child should empty their bladder again just before sleep.
Just keep using bedwetting pants and wait until you find them dry a few nights in a row, or you have medical treatment given.
It really will get better in time, don't make them feel guilty for it whatever you do, as it is beyond their conscious control. I wouldn't recommend waking them to take them to the loo before you go to bed, or putting them on the loo while still half asleep, you just get tired grumpy kids who may still be wet next morning anyway. Don't let grandparents bully you into doing this, and don't compare your DC to others who were dry at night much younger, it really isn't helpful.