Hi Stef and Steve:
Wish I could say we were joining you (we have two kids too). We did a 14-day Transatlantic out of Barcelona last September and had a wonderful time. Is this your first trip to BCN or Spain?
We did a 4-day pre-cruse in Barcelona. The hop on / off bus was worth the money for us. When we arrived in BCN, we did the HOHO bus the next day, to get our bearings. The day after that we stopped at the places we wanted to see: La Familia Segrada, Montjuic, Gaudi's Parc Guell, etc. All were really amazing.
If you guys have the time, take the train to Montserrat. We did it on a Sunday. It's a magical experience. Depending upon how old your kids are, they would really enjoy the tram and funicular ride. The oldest boys choir in the world sings every day (I think) at noon. Great place for a picnic too.
Hope you guys have a great time!