They have a great tourist bus that has 2 routes around Barcelona (one was better than the other, I can check which if you ever need to know) and you can hop on and off the route. Good way to see everything without getting worn out if it's hot or the children are getting tired.
Must admit we only ever hit school holidays once when we used to got Portugal/Spain with our two little ones...............they start at the end of June I seem to recall.
Worst thing is that both the Spanish and Portugese kids have no concept of the word " queue "...........as soon as the gate opens for a ride they'll push straight past you with no embarrassment or care whatsoever while their parents look on full of admiration at the little brats. It amazed me that what are generally very polite children turn into uncontrollable little horrors once they see a chance to get past anyone and anything between them and a ride!!!!!