Do not pack light regarding clothes and shoes. Pack lots of medicine, as there is some disease in Vietnam. Though you can buy clothing in Vietnam, none of it will fit you. The Vietnamese love westerners, be prepared for lots of questions and your children will be adored, especially if of fair complexion as it is so different. The roads are chaos: thousands of motorbikes carrying families, fridges, pigs- its madness. Also, crossing the road takes extra care as bikes do not stop; they go around you. Not for the faint-hearted. It will be very hot and humid especially further south so bring hats and lots of water (6000 dong for a liter bottle is a good price - about 15,000 dong to $1 and you can use both currencies). Haggling is good fun and even water is to be haggled for. Always approach it with a smile. Wet wipes/anti bacterial dry hand wash is always recommended and take a toilet paper roll for when you are out and about. Bring on portable bathrooms if you are prepared.
On road trips, ex. from Dalat to Saigon or Nha Trang to Dalat, stop by smaller city as you are on the main Highway. The markets range from Tea/Coffee to Pottery to handweaved bags. Make sure you have your camera handy for there will be sights that will amaze you; ex. a family of 5 riding on a motorcycle.
Hoi An is very safe. If you love shopping then this is heaven. You can have shoes made, or suits and summer dresses made. All are very good quality and inexpensive. Go for a deal where you 'don't pay until happy' (or only pay small deposit) Yaly is excellent for clothes, although they are more expensive and it is more fixed pricing ($25 dress, $120 top quality beautiful suit). There is a lovely French coffee shop by the river and the older Vietnamese here do speak French. There are also beaches nearby.
Halong Bay, only a few hours out from Hanoi is not a great place to stay as it is being developed and there is not too much there. However, a boat trip out from Halong Bay is a great thing to do and the cave exploration will thrill the kids. Bai Tho is a good outfit to hire a private boat and see the bay for 4 hours. Kids who are enjoy nature will love the view and appreciate its beauty. Kids who enjoy being on boats will just love the experience.
As of June 2006, there were more hotels developed and lots of tourists enjoying the amenities and beauty of Halong bay. Rent out a junk to stay overnight for it is an experience to just enjoy the quietness and beauty of Halong Bay. A must visit if you have 2 days in the north. Hanoi is a great, very safe city. Be prepared with water bottles!
Hue is beautiful and you can hire bikes/motorbike rides out to surrounding scenery. Hue may not be as fun for young kids because it is mostly cultural and historic.
The overnight train to Hue from Hanoi is very cheap and has cabins of 4 drop down beds. Cyclos are nice to ride in.
HCMC (former known as Saigon) is not as safe or friendly; definitely stay in the tourist areas. The cu chi tunnels and historical tour (just out of HCMC) is well worth going on and will fire the kid's imagination. The war museum in HCMC is not recommended for children since its photography is explicit and it is unblinking in its coverage of the Vietnam War.
Vietnam has some nice hotels but bear in mind that it is not a rich country, so it is not western style luxury. In Hoi An the Victoria or Golden Sand are right at the beach, and are convenient for families. Golden Sand has a nice, long pool where the children can play.
For a nice beach, Mui Ne is recommended. It has a nice long stretch of beach lined with a variety of boutique hotels. A road runs parallel to the beach with plenty of restaurants and a few shops. It's a fairly low key resort, but absolutely lovely. There are some spectacular bright red sand dunes there then you can sled down on bits of cardboard, great fun! The wind can pick up on the beach in the afternoon, the breeze is great but you can end getting sand blasted so it's probably be best for the kids to be on the beach on a morning. Also recommend is the Sailing Club Mui Ne.
There are companies that run specialist family adventure tours as well as tailor made holidays in Vietnam (and elsewhere in Asia). This kind of holiday is intended to take all the hassle out of traveling in a new destination and ensure that the itinerary that you take on is suitable for every member of your family. You have the assistance of a guide when you need one and the convenience of hotels (family friendly) booked, air conditioned transportation, fun meals for the kids to enjoy and activities that are ideal for the necessary age group - these could include kayaking in Ha long Bay, trekking in the central highlands, mountain biking and visits to the cu chi tunnels.