Carys,
The are quite some nurseries here that are not bad at all,my two years old daughter go to one called "Elder Tree" which of course I think is the best one you can find here,the management is british so this should be an advantage to you I hope.
As for KGs I beleive most start at 3 years old.

Lea,don't you have intentions to enrole your kids in a school here??
I understand how confused you are,I've been there...but just don't worry,the people here are nice and kind and you will find a lot of help whether on this community website or in reality when you come over here.
By the way what part of the US are you from?? I used to live there for 4 years.