Book your ferry ticket on the Tallink ferry online before you go, they have the newest ship, and it's very kid-friendly. Tickets can be changed for a small fee if you need to come back earlier or later.
Estonia is very far north, so it can be cold! Summers are typically cool, not hot.
Take the earliest ferry from Helsinki to Tallinn - The earliest ferry leaves at 7:30 am. If that is too early, you can also take the 10:30 am ferry and still have plenty of time in town.
All seasons are great, but if you visit in winter, expect snow! Summer is mild, a light jacket/sweater will likely be needed.
Book ferry tickets as far in advance as possible. You can change the times if needed for a small fee, but the best deals tend to be with advance reservations.
This is the perfect itinerary for a day trip from Helsinki with kids. Your entire family will love walking through Tallinn’s historic town and making up stories about who walked there before you – what did they do for a living, where did they come from, and why were they here? The city is a perfectly preserved medieval town that is comparable to Dubrovnik, Croatia (architecture-wise), and Colmar, France (quaintness-wise). Since it is a less-traveled destination that has not been overrun by tourists yet, you’ll get an extremely authentic feel, unless there is a cruise ship in port the day you go. Enjoy this fun-filled day but be aware that it is a long adventure (especially if you are coming from Helsinki by ferry). Try the stroganoff in any of the restaurants in town; they’re famous for it!