I think that fit in shoes is very personal and can vary with manufacturer, probably because of the lasts that they build the shoes on. I've always been comfortable in Merrells, but they always seem to discontinue the shoes that I like.

It's probably not the right time of year to look for merino wool socks. Walmart had some inexpensive ones in the sporting goods department that fit and wore quite nicely, but I didn't notice whether they had them in kid's sizes. I have worn acrylic socks for years. The most recent ones were Dickey's steel toe boot socks. Again, I haven't looked in the children's socks, so I don't know what is available. I would look for a blend that is mostly acrylic with some nylon.