Just found a weeks Bed and Breakfast ski holiday from 19th to the 26th December For £170 each if 4 are sharing.
It is staying at the Poblado apartments in Arinsal in a 1 bedroom apartment. Trip advisor reviews suggest it is more for groups or families with teenagers than young kids.
Flights are from Gatwick or for £25 supplement from Birmingham.
The accommodation includes breakfast and linen and is very near to the slopes.
I have checked out lift pass prices and for that week it will be 153Euros (approx £140) in resort, Inghams are offering it for £144. Ski hire from Alpinresorts.com using promotional code news101 is 54.60Euros (approx £50) inghams are offering this for £64
I don't think you will get a much cheaper ski week especially at Christmas. I have already booked a week in the New year all booked independently and it has cost about £380 each self catering including skis and lift pass this would be about £360 each with skis and lift pass.