With are those boys wearing?
The kids in the background are part of a choir (combined for the occasion from pupils from a school in Best and the Cathedral Choirschool in Utrecht) that performed during the opening of this national Education Exhibition (or Convention). Some were wearing a baseball jacket, some a denim blouse, and some girls were in a blue polka dot skirt with a white blouse. It's pretty much kids' choirs' standard dress code, courtesy of Kinderen voor Kinderen.
(For the last 37 years, the Kinderen voor Kinderen (kids for kids) choir, that is newly formed every year, sings songs composed by our 'best' composers and writers. These songs cover issues that are important to kids (and that are submitted by kids), from fun stuff like having fun, holidays, dancing and being in love with a popstar to serious stuff like divorce, being bullied, money (and money trouble), sadness, loss of relatives (and pets) etc etc. They provide the 'anthem' for the Koningsspelen (the gamesday at schools to celebrate King's Day), National Book Week and sometimes a charitable occasion. Check 'Kinderen voor Kinderen' at You Tube for the craycray apparel they've been wearing through the years)
or check kvk.vara.nl/koor