Ive name changed only as I've discussed this elsewhere.
I've been with young children. It's a very child friendly destination, although it can be a bit too friendly at times if you have a blonde or red haired child. Just good natured interests though. Halong Bay is definitely a must see. I loved Hoi An (good for cycling and cooking classes) and Saigon, didn't much rate Hanoi. Loved the trip to the Mekong Delta. Hue was a mixed bag. Have you been on a night train before? They're an experience. DC's favourite activities were the Halong bay trip (we did an overnight cruise) and the water park). I wish we'd been able to do a homestay, but it wasn't possible, although we did have an amazing home cooked meal. Try looking at some of the travel sites that run family trips to vietnam for ideas of itineraries, although you might want a more relaxed pace. Think Intrepid, Adventure Company and Exodus, from memory.
We didn't go to Sapa, but I have a feeling anti malerials are required there so that might be something to consider. Presume your kids are up to date with vaccinations, but if not can recommend the STA travel clinics as they definitely vax children. NHS couldn't obtain the right vaxes, even though we were paying for them.
Visas - everyone on the trip advisor forums (which are very helpful), will tell you to get the cheaper visa on arrival. We paid the extra to get visas by post from the embassy in London before we went. Very glad we did as the queue was enormous for VOA.
Money - the trip advisor people will tell you not to get cash before you go as the exchange rate is awful outside of Vietnam. Personally I would recommend getting at least some Dong before you go just to be on the safe side or take a reasonable amount of your own currency. We had a heartsink moment at the airport when none of the cash points would work and we realised we'd forgotten to bring any cash. Did eventually find one that accepted my card, but it was anxiety we could have done without.
Roads - the thought of crossing roads in Hanoi still gives me palpitations. The trick is to keep moving. If you stop then everyone panics.
If you're flying from the uk I think I considered just about every flight option. Bear in mind there are very few direct flights (and those I know who flew with Vietnam Airlines didn't really rate them for service) and you'll have to fly into or out of Hanoi or Saigon. I don't think Da Nang is a direct flight. You could break up the return journey with a stop over elsewhere in Asia? I think you could Easily do Bangkok, Hong Kong or Singapore?
I'm so jealous, I'd love to go back.