A 30€ brunch in Porto usually includes quite a lot - main course, drink, dessert, coffee - so maybe it's not that different from going to a regular restaurant and paying 15€ for food and 5€ for coffee separately. It's worth it if the place has a good reputation and people who've been there recommend it to you, because sometimes it really is about quality and atmosphere, not just hype. The real question is: will you and your daughter actually enjoy the experience and is the food genuinely good, or are you just going because it's trendy? If it's the latter, there might be cheaper alternatives that would make you both just as happy.