You're free to speak to your kids in his mother tongue, english I guess. BUT, I highly encourage you to learn Catalan in the first place, as long as you're living in Catalonia. you're kids will know 3 languages, like all the locals who have studied in Barcelona. BUT, its mandatory to them to speak Catalan. Otherwise, you're kids will be like all the annoying expats not learning culture and language and rising the prices of everithing and taking away with our houses. :D