Mustasaari is a lovely place to go to in Helsinki with children! It is an island owned by the Lutheran Church but free for everybody to use. However, the return trip on a boat cost 4e/adult and it is a 15min ride to get to the island.
Boat Walhalla pics up passengers to Mustasaari island from this jetty at the end of Eteläinen Hesperiankatu:
In my opinion the island is well kept, and has great facilities for families. There are picnic tables both with and without cover (loads of cover in case of rain), toilets, potties, changing tables, bbqs, a playground, some chooks and bunnies in cages as well as sheep that like to be tickled behind the ear in a paddock. There is a café (below) that serves lunches as well.
The beach seemed to be shallow and therefore great for kids to play in. Unfortunately we didn’t get to test the beach though, as we did not think of packing our togs (BIG mistake as our 3yo told us). You can also cross over a tiny wooden bridge to Hevossaari to have a bit of a walk in the nature.
I found the boat and island trip worth taking already for the scenery as I just love the Finnish archipelago.
Mustasaari seems to also cater for various courses and shows that cost separately. When we were there, some youth were about to show some circus tricks.
Mustasaari island is alcohol and dog free, and you can only visit it during open hours (Mondays closed, opens other days at 9am and closes at 6pm, Wednesdays and Thursdays the island is open until 9pm). The ferry leaves from a little jetty next to Hietasaari beach and grave yard, and there is a fair amount of free parking space by the jetty. The ferry leaves approximately once every hour, please find the schedule here.