The age limit for a child to wear diapers in Daycare is usually about 4 years old.
Most Daycare providers have a policy on when a child must be potty trained to continue attending their daycare.
Age 4 is usually the cut off age for kids wearing diapers full time to Daycare.
Also if your child is in the midst of potty training then they may require the child to wear diapers when in Daycare for at least a few weeks to a month to prove they can keep their diaper dry.
If the child can keep their diaper dry for a few weeks to a month and has no or little accidents then they can wear underwear.
The daycare I send my 2 year old too required my 4 year old to wear diapers for a month after potty training to prove he could keep dry.
Then he could wear underwear.
Kids of 4 years old can be harder to change diapers on so Daycare providers usually have an age limit and if not potty trained they may terminate the daycare contract with you.