I finished my first mitten ever today.
I also used the afghans pattern, or at least that was my intentions. I tried using one strand of worsted yarn and the mittens seemd quite thin when thinking of the - temperatures you quoted in your post. I double stranded the yarn and used a 4mm needle cast on 24 stitches knitted flat for about 1-2 inches and then the same again in the round in 2x2 rib. increased two stitches and then knitted the hand with 26 stitches. I decreased like you do for a sock and did a 3 needle bind-off. Thumb i did as instructed. the glove is quite a firm and thick fabric more like a felted fabric. thought it would take a bit longer for the wet to soak through. The glove is a little big for my 2.5 year old but to small for my 7 year old, so imagine it will fit a 3-4 year old.
wear it long to keep the wrist/arm warm or folded.