Hi!
I am a size 2-4 here (US) and 38-40 in Spain so a 6 is a 42, a 8 is a 44... But I find that sizes is Spain are smaller.
You'll find most of the tags with European sizes and some of them with US sizes too. You can go to Zara too, all their tags are in different sizes (Spain, Europe, US) and currency.
For shoes I am not sure, but I beth they will be more than happy to let her try different sizes.
Have fun!
Nuria