The Parc de la Ciutadella is probably one of the most famous park of the city. Built during the middle of the 19th century by Josep Fontseré on the ruins of the former Citadel, the parc has been during a long time at the heart of the life of Barcelona. For example it hosted the 1888 Universal Exhibitionand welcome in several buildings from the former Citadel (the Chapel, and the Arsenal which is now the seat of the Catalan Parliament) and from the 1888 Universal Exhibition (The Castell dels tres Dragons which was a restaurant, the waterfall, the lake...).
The Parc de la Ciutadella is the ideal place to have a walk and rest. With the arrival of sunny days you can enjoy a boat ride (3€ for 30 minutes for 3 people). Moreover go to have a walk in gardens of the parc, there are more than a thousand of species which are among the oldest of the city (some of them are from the 19th century).
Don't hesitate to admire the splendid architecture of the Parliament, the waterfall (made among others by Gaudi) or to discover the severals sculptures.
Finally, the zoo which occupies a large part of the parc will be a delighfull moment for you and your children!
More info: Barcelona Tourism - Parque de la Ciutadella