LOL, just realized I somehow undesrtood a 8 years old child, not twins of18!!!!! Of course you will not have any problem with them going to any bars... and, please, let them drink a bit, but only the good stuff! :))
Can Paixano and El Xampanyet are not places to sample good cava, not even half decent cava. They may be interesting places for other reasons and a real experience indeed
Mind that here "xampany" is a local old-fashioned way to call cheap carbonated wine. It has litte to do with autentic Cava or Champagne apart of the bubbles! :)
La Vinya del Senyor is a good wine bar. I don't know about the other 2
We locals usually drink cava withl meals, rarely alone. Mostly where celebrating something (even if just celebrating that's Sunday or that we feel happy). Many good cavas taste much better with certain foods. There are many kinds of cava, here we tend to like very dry ones (brut/brut nature) but there are half-dry and sweet ones. If you don't know, go to a good wine bar (or restaurant) and ask, explain the sommelier your tastes and let them to guide you. Many people who taste cava and do not like it, may have just tasted the wrong choices!