Updated June 2011: Isaac is adorable and suits the part of Brandon Stark incredibly well. Very pleased with this actor playing Bran.
The only kid I could think of back then who would've suited Bran while I was reading the books was Thomas Sangster.
Catelyn and Eddard's second son, Bran enjoys climbing and exploring Winterfell. Like his siblings, he is dutiful and tough-minded. He is named for his deceased uncle Brandon, elder brother of Eddard. He dreams of one day being a great knight. His direwolf companion is named Summer.
I imagined Bran as one of the kids from Nanny McPhee - Thomas Sangster. While I was looking for photos of him, I realised he's grown up quite a bit. He's almost past his teens now and no longer that little boy. Oh well...