It could easily be quite cold - under 10c during the day and below freezing at night.
You can but very good quality cold weather gear in Hanoi at about 10% of the price you would pay in the US.
Buy a sweater and a coat each (under $50 for both of you) to use during your time in Sapa (Hanoi can also be cold)and then give them away when you are leaving Hanoi - maybe to KOTO, Hoa Sua or some other "streetkid" NGO.
If this idea appeals to you I can give you the address of a good coat shop and somewhere to donate.
And make sure your hotel has heating. We stayed there a few weeks ago and although the beds were quite warm, a heater was extra. Also if there is a fireplace check what they burn, they have small cylinders of black "material" not sure what it is but it smells pretty bad
We too were there recently. We took an old pair of trackpants and a jumper each. Clothes we had no attachment to. Nobody cares what you look like. We wore them on the night train, then we left them behind in Hanoi as Martin suggested. One tip though, if possible leave your large luggage in Hanoi and just take an overnight bag. We did and it made life a lot easier.
we will be in sapa the first week in april. What about the weather then?