A child, will have accidents.
Even if they are already potty trained and already older.
Then there is night-time dryness. Which does not occur, biologically, until even 7 years old. And this is normal.
She is not ready, for underwear.
She has to be ready.
She has to have her biological development, in place first.
There is the brain/bladder/nerve development, which impacts when a child can attain, potty mastery.
Accidents will occur.
Whether ready or not.
Potty training takes time.
And will take much more time, if the child is not ready, emotionally or biologically.
It occurs, in stages.
Not in one swoop.
By the time my son was ready, to go out in just underwear, it took about 6-8 months, of him being in it just at home.
Any kid, if in a pool, wears swim diapers. Until they are completely potty trained and don't need it anymore.
Wearing underwear, will not make a child dry or potty trained. By itself.
It is a stage, to progress toward. Not just put on, and then the child is trained.
Kids, even in Preschool, Kindergarten and 1st Grade, still have accidents, even if potty mastered. I know, that is what ALL of my daughter's Teachers, have said.
It is their biological maturity... and then the child's ability, to know their biological cues, and then being able to time it (their body) along with how long they then can hold their bladder/bowels, before rushing to a toilet.
My son is 4 almost 5. And he is still in diapers for nap and bedtime. He is still wet, during sleep. It is normal.
My daughter, was still in night time diapers even at 5 years old. And this is normal.
Kids will have night-time accidents. Until their physiology is matured.
My kids do NOT get 'confused' about the need for diapers at night. I simply tell them their body is not ready to be dry at night. No biggie. They understand. They understand the progress... of their BODY, developing.
Because I explain it to them. Not just expecting them to be dry and accident free. All the time.
It is a biological, attainment and development.