I would do 8 nights as well. We're still away on a long trip with our toddler that started in Barcelona, and jetlag was really hard on him. The first few night were pretty hard.
Love of my life baby boy born 11/11. One and done not by choice; 3 years of TTC yielded 4 MMC and 2 CPs, through 4 IUIs and 2 IVFs. Focusing on making the world a better place instead...and running.
We also did Montserrat but I wish we had time to rent a car and go to a winery an hour away from BCN. I would also have loved to check out Sitges, but not sure if that's kid-friendly.
If you do rent a car,parking is expensive. We had a rental car there and all the parking garages were around 30 euros a day to park. We stayed in the Gothic Quarter.