Get a few waterproof bed pads.
Then that way, the sheets will not get soiled.
I got mine from Amazon.
Night time or nap time dryness, is a physiological occurrence.
Not per age.
Night time or nap time dryness is not something that physiologically occurs until even 7 years old and is normal.
It has to do with the brain/bladder/myelin nerve sheath development.
Day time pottying and night time, are two totally different things.
My son is 5 and is still wet at night and at naps. And I use Huggies night time diapers. It holds more than pull-ups.
My daughter even at 7 years old had night time accidents.
Normal.
My kids, NEVER got "confused" about wearing underwear during the day and night diapers at night. Because... I simply explained to them that their body is not ready yet to be dry.
And they understood.