Marienlyst is one of my favorite spa hotels, although it is in Helsingør, not Copenhagen. If you have a bit of a longer trip here and have time to get out of the city, I highly recommend an overnight stay at Marienlyst. The hotel is located right on the water (I think it's the Øresund Strait) and much of the hotel, as well as the spa, overlooks it. The entire hotel is beautiful, but the spa is the highlight!! (I couldn't find an official age requirement on the hotel's website, but I have seen children there before. I am sure all ages are welcome in the hotel, but I would only bring older/very well behaved children to the spa out of consideration for the other guests.)