They are all easy enough to travel between. Depending on your time (1 or 2 weeks, or more?) I would certainly try & include some time on that end of the Peloponnese too. Basing at or near Nafplion (Tolo might be good choice for the kids for a beach day), you have a variety of sites nearby.
What are your priorities for your time? History, sites, walks, beach etc? How 'young' are the children?
End of July into August you definitely need to get most, if not all, your accommodation sorted. It is peak time for Greeks to be on holiday & the proximity to Athens means they will be very busy.

Hi, Thanks for the info. The children are 5 and 2. I was thinking about the accommodation, and have had a little look around, but I didnt see that much really. Ill keep looking. Any more info much appreciated. Im going for 19 days altogether.

Hi Joey,
we have spent time on these 3 islands and particularly liked Poros. It has some good beaches and really lively main town. We stayed in the Saga hotel with lovely sea views and a pool. The owners have a farm on the mainland and will take the children on trips to see the animals.