All hotels will usually have a Greek night, usually really worth watching. I was 7 and 8 the two times I went to Kos and I enjoyed myself a lot, they were four of the happiest weeks of my childhood.

There's a huge castle by the harbour in Kos Town, definately take them to see that.

There are a number of boat trips to be had around Kos e.g . you can catch a boat from Kos Town to Kardamena where you have a chance of spotting a dolphin or two, there are day long boat trips that visit 3 other nearby islands from Kos Town which are good value for money especially if you book directly with the boat themselves and not through a rep, you can catch the Flying Dolphins boat service to Bodrum, Turkey which takes about 40 minutes and again is a relatively cheap excursion and a good day out, hope this helps.