My feeling is that with that amount of folks and spread of age groups whose needs are to be catered for you will need one of the more popular Islands that have had established tourism for many year.

Corfu, Rhodes, Crete or possibly Kos spring to mind. Each of these Islands will have various different styles of resorts to choose from, the very busy resorts to the quieter resorts where you will still be able to get access to the busier places should you so wish.

You probably also need to decide if you want self catering, half board, all inclusive etc

Doe

zante is nice.don;t forget you will and might have to pay extra for air cond.and some places in greece are not up to euro heath standards.like spain.the hospital are poor in some places.and don;t forgot to put used toilet paper in the bins.