Ha Long Bay is a few hours out from Hanoi. However, a boat trip from Ha Long Bay is a great thing to do and the cave exploration will thrill kids. Kids who enjoy nature will love the view and appreciate its beauty. Nha Trang has made great strides lately and also has a nice beach. For kids, the main attraction would probably be Vinpearl island theme park, reached either by boat or cable car. There is also plenty to do around and about. Very good service quality with a warm and nice welcome.