Children aged 6 to 18 do need a certificate to travel just like adults. As long as they are not vaccinated they can get a recovery certificate or test certificate. Once they’ve been vaccinated they will receive a vaccination certificate, just like adults.
Please note that each EU Member State is free to set different conditions for travellers aged 6 to 18. The conditions of all countries (and often separate regions) are available on the ReOpen EU website.