That’s a lovely idea! All the best to your friend, I hope everything works out as smooth and uneventful as possible ❤️
But I gotta admit I’m far from a children expert as well lol. He was about two at the time, pretty talkative and running around like a madman. So my part was pretty easy, I did a quick internet research for average foot sizes per age and knitted them a tiny bit bigger, so he could grow into them. And the rest he literally just told me 😄
Though I’m pretty sure for actual babies tube socks (without heel) are more a thing. Once because they grow so fast but also because they do completely other stuff with their feet than us walking people. Makes sizing way easier as well.
Also for favourite things: It’s more an educated guess, but I’d say as soon as comprehension for those things kicks in there’s almost immediately a favourite. There’s always that one pencil, toy, shirt, person they just like more than others.