I can highly recommend the Tower Restaurant, located in the National Museum building. It may be a little too grown-up and haute for the boys -- depends on their tastes in food and how comfortable they are in a white-tablecloth restaurant. We had a lovely dinner there and just raved about the fantastic view of the skyline (including castle) as the sun was setting. Take a look at the menu if in doubt about how suitable it may be. https://www.tower-restaurant.com/menus
Even though it comes across as a dress-up place (and my husband did wear a sport coat, but no tie), we saw families there wearing jeans, sneakers, and hiking-style parkas.