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.
Consult the website of the regional governments for the rules regarding the Covid Safe Ticket: