The people at the Palau de la musica are very friendly (which is not always the case in Barcelona I'm afraid to say). It's a beautiful building. The tour is about 45 minutes (starting with a 10 minute film). We did the tour with a toddler which is very doable. You can keep the stroller downstairs (in a locked away place) as you climb the steps of the building.
It is expensive at 17€ per person for the visit (you get a discount with the Barcelona card). When we visited, there was a maximum of 55 people per tour. At the tour itself, there were only 20-30 people there. I think during the week, it's relatively calm. Even so, the building itself is created to comfortably hold an audience of 2048 (see, I learned something) so I wouldn't worry if the tour crowd is a bit bigger.
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