Generally speaking, it is not necessary for a baby to wear underwear when wearing training pants. Training pants are just like underwear. If the baby wears underwear again when training pants, it will be repeated and it will become more troublesome to change. If the baby doesn't need to wear training pants, he should put on underwear. Generally, the best age for babies to wear underwear is 2-3 years old. Wearing the right underwear at the right age is good for the baby's growth and development.
Training pants are suitable for babies over one year old. Babies at this stage generally have a certain awareness of urinating. The training pants are designed to help the baby get rid of diapers as soon as possible, and can also help the baby to open the habit of crotch pants. But training pants can't be worn as autumn pants, and babies have to wear pants after wearing training pants.
In short, when the baby grows up slowly, parents should constantly remind the baby to urinate, so there is no need to wear training pants for the baby. When the baby doesn't wear training pants, it is necessary to wear underwear for the child.