An outdoors sort of family destination — beaches, a castle mound and a farm museum rather than attractions — with one reliable indoor fallback in Kuressaare when the weather turns.
Kuressaare Castle grounds and moat
The castle at Lossihoov 1/1 opens daily 10:00–18:00 with last entry at 17:30 and tickets at €9–13, but the water-filled moat, the ramparts and the surrounding park cost nothing and are the part small children take to. Easily an hour either side of the museum.
Mihkli Farm Museum, Viki
A complete thatched farm complex built up by eight generations of one family, in Viki village on the west of the island. Open Tue–Sat 10:00–17:00 with the ticket office until 16:30 and tickets from €6. Outbuildings, farm tools and room to run about.
Mändjala beach
Shallow sandy beach backed by pine forest about 10 km west of Kuressaare, with campsites strung along it. The water stays waist-deep a long way out, which is exactly what you want with small children. Free; bring your own food, as there is little on site.
The water and sauna centre at Grand Rose
Tallinna 15 in Kuressaare, open to day visitors Sun–Thu 09:00–21:00 and Fri–Sat 09:00–22:00. The dependable wet-weather option in a town that does not have many; check prices and availability through the hotel's spa booking before turning up.
The ferry crossing itself
For most children the Virtsu–Kuivastu ferry is the best part of the day: open decks, gulls and the mainland sliding away. Foot passengers and cyclists board without a booking, and if you are in a car it is worth getting out and going up top.