Dublin isn't the largest city in the world, which can be a blessing for younger visitors. It's easy to explore much of the city centre on foot, or jump on a short journey by bus or tram.
Likewise the city's main museums are manageable in size, from the Natural History Museum (aka 'the dead zoo') to the Science Gallery, a contemporary addition to Trinity College with centuries of history. There are some perfect for families, such as Dublinia and National Wax Museum Plus, with plenty of interactive exhibits where fun meets learning.
Dublin is known for not having the best weather, but in addition to the indoor attractions, there's plenty of outdoor fun to have should the sun shine. In the city centre there's St Stephen's Green, with summer recitals in the bandstand, and the expansive Phoenix Park where there are bikes for hire for all ages.