IMO a child will night train when their body is physically mature enough to hold urine all night long. Hereditary can also play a role in bedwetting too fyi. I think using the toilet before going to sleep and limiting fluids can only do so much but is a good idea! Some kids sleep deeply while others seem to wake when they feel the urge to urinate. Lots of time and patience is key! I would put some type of waterproof cover on the mattress
