Ephesus

I can't tell from the wording of your question if you want to know the distance from Kusadasi to Ephesus, or if you want to know the size of the Ephesus site. The port is 15-20 miles away from Ephesus, plan on maybe 30 minutes between the two. Ephesus itself is not all that large, so you don't have to be choosy about what to see. Terrace Houses are an extra TL15, payable only in Turkish Lira, on top of the TL20 Ephesus entry fee. Ephesus is on a constant incline/decline, mostly on stones. Many people choose to start at the upper entrance, walk downward through Ephesus, end at the lower entrance, and maybe enjoy a snack outside the gate, where there are always several taxis waiting.

Taxis should not be a problem to get, but you will need two because of the size of your group- unless a larger van is available. The archaeological site is orientated "top to bottom" roughly meaning a downhill walk if you start at the top. Guides are at the gates who speak English and can take you around. As previously stated, the terrace houses are an extra cost, but are exquisite. I have been to Ephesus several times, and for me, on our last visit, the houses were the highlight of the visit. They are generally much less crowded than the rest of the city and are just really cool, literally as well as figuratively since they are covered! With the ages of your kids, I think the site is about all you and they will be able to take. I would tour around, go back to the boat for lunch, and then anybody who wants to walk around Kusadasi can do that after you eat.

A word of caution when walking through the site. There is a considerable amount of marble here, which means it can be quite slippery when walking on the foot path. Be sure not to let your kids run around too much or they may take a nasty fall.