First of all, take your children to the local zoo, the largest in the country. It is interesting because the animals here are a perfect imitation of their natural environment.
There is no such thing as an amusement park in Gdańsk, but you can go to another city of the Tri-City, Gdynia (22 km from Gdańsk), which has the largest amusement park in Poland. There are rope courses, off-road vehicles, a shooting gallery, paintball, a climbing wall and many other attractions for guests large and small.
And just 12 kilometers from the city is Sopot, where there is a very good water park. And, of course, a child should be taken to the aforementioned amusement park in Gdynia.