A pair of mittens takes 1 hour per mitten * 2 mittens = <<1*2=2>>2 hours.
A pair of socks takes 1.5 hours per sock * 2 socks = <<1.5*2=3>>3 hours.
So a complete outfit for each child takes 2 hours per hat + 3 hours per scarf + 2 hours per pair of mittens + 3 hours per pair of socks + 6 hours per sweater = <<2+3+2+3+6=16>>16 hours per outfit.
So to knit winter outfits for all three of her grandchildren, Martha will take 3 outfits * 16 hours per outfit = <<3*16=48>>48 hours.
#### 48