Hi,
We have never stayed at the Athina, but they look very nice.
Our daughter is 8 and we have been many times to Kalamaki, unfortunatley entertainment is limited for kids there.
Your best bet is to use the pool at Antonis it is just a bit further down the track by the mini golf. There is usually plenty of kids there and they are made very welcome by the owners(plenty of sunbeds to laze on to!) they also have birds and a donkey which the kids love. They have entertainment some nights as well which is worth going to.
Have a lovely time, we are trying kos this time hope its as nice as Zante!