When we've been to these parties, the invite always says- all participants must wear socks (including parents) but I still think people forget - especially in summer, so this is a cute idea. For adults, I would vote for just a bag of white athletic socks for men and women (2 styles) and not hand them out as favors but have them there in case anyone needs socks. I forgot socks once (i didn't think dd would want me to go in with her but she did) and I was grateful my friend had brought parent socks! I like Target for adult socks - I am not a fan of "cute" adult socks and DH is definitely not a fan of them!