Firstly, it is definitely cheaper to go free and easy.
To go to Sapa, you can book tours in Hanoi. However, often they will con you money by charging you 1st class sleeper train tickets but only offering you the 2nd class. trust me. travel by first class is better cos 2nd is really squeezy.
To avoid such incidents, always go to the train station and buy tickets yourself. Do purchase maybe a few days ahead if you really wan the 1st class sleeper.
Also, there are tour agents in Sapa who provide tours, trekking routes which can cater to your liking and degree of difficulty. Hanoi travel agents wouldn't be as knowledgeable. Another plus point is Sapa tour agents would charge cheaper than Hanoi agents.
From Lao Cai train station, you need to take a minivan to Sapa, takes around 50mins to 1hr. the road is windy and mountainous. Generally, if your child has no motion sickness, I think it would be alright.
Carrying your kid around might be a problem if he or she is tired during trekking. cos what you really wanna do in Sapa is to trek with the Hmong tribe ppl (tour guides are Hmong and they speak way better English then Vietnamnese) and appreciate the mountains and the rice terraces.
I went to Sapa in Dec as well. To be frank, the locals said the weather changes quickly, there isnt a fix season for foggy weather. I went 3D2N and 2 days were Sunny. Really depends on luck. What I can say is to avoid the rainy season since you are more likely to slip and fall and concern about your footing when doing the trekking.
If you are going with the family, maybe its better to pay slightly more to ensure things goes smoothly. After all, having fun its more important then the extra money.
Hope it helps.
Personally I really like the Lao Chai Tavan Trek. (note Lao Chai village is not Lao Cai which is the train station)
