Most airlines allow unaccompanied minors, you need to get the special unaccompanied minor tickets, sign them in at boarding and have an adult collect them and sign them out at the destination airport. Once you’ve signed them in the airline hangs an ID pouch containing their tickets and passport around their neck and a stewardess accompanies them to check in and for the entire flight, handing them over to your nominated adult collecting them at the destination airport upon production of photo ID. That’s how we sent our 11 year old off to France with Air France in the 1980’s to stay with friends for the 6 week summer holidays.
Different airlines have different unaccompanied minor rules most seem to be for 11-16 year olds but it varies. The market leader as it were is Norwegian Air who appear to allow 5-15 year olds as unaccompanied minors. Check with the airlines covering the various routes to Greece.
Don’t forget, if one parent accompanies them out and the other accompanies them back, you need to make sure their names match on passports. Me and my partner are not married and we decided early on our daughter would only use one of our names for her simplicity, we didn’t want to burden her with a double-barrelled encumbrance. It’s meant more paperwork for us when the parent whose name she is not carrying is travelling alone with her.