Maybe some of the more experienced knitters can give some input here. Please correct me if I'm wrong, but I question the use of sock yarn with the size needles called for in this pattern. I think that sock yarn would generally be used with a size 3 or 4 needle. You would have to buy an awful lot of sock yarn to do this whole set, and that can be expensive.
If I were doing this pattern, I would use baby sport weight yarn for this, like Bernat baby sport weight. Just the past few months, I bought this yarn for some of my projects. It comes in large skeins, can't remember the exact amount because I threw the wrappers away. But I think you could get this done with that yarn more economically. And this Bernat is designed for needles of the size called for in the pattern. I believe you could safely substitute this. And the plus side is that, since the yarn is acrylic and not wool, it could be machine washed. New mothers don't always have a lot of time when they're caring for a new baby.
Edit: My thinking is that a size 6 needle might not be appropriate for sock yarn. It may make the work too thin or holey looking. Sock weight yarn is like a fingering weight. Baby sport is like halfway in between a fingering and a sport. A little bit heavier. You might have to go down a needle size for the baby sport. You'd have to check your gauge. What do you think, fellow knitters?