Have a good stay!
You can easily spend several days in Moominworld and other local family attractions. It is advisable to book accommodation in advance, especially in the middle of summer.
There are plenty of good alternatives for accommodation in both Naantali and Turku. For example you can choose a relaxing spa hotel or a child-friendly family hotel. Some hotels offer handy Moomin packages. More information can be found on the Naantali tourism and Turku Touring websites.