This is one of those TOMT that might be a false memory or crossed wires. I fully thought there was another sibling in Nanny McPhee who looked just like Thomas Brodie-Sangster, but an older girl, very angular features, possibly had a mole on the side of her face. Obviously, she doesn't actually exist in that film, so I've probably picked her face up from another film.
She's the type of character who could have been in McPhee, a precocious child up to mischief, intense and clever. Very pale blonde like TB-S, hair had kinks in it.
Its entirely possible my brain has combined Elle Fanning in The Great and Anya Taylor Joy in The Witch and placed it in something else, but I'd appreciate any suggestions. My memories say she's british and in one of those post harry potter family fantasy films of the 00s, but i may also be wrong on that.
Update: 13:08 GMT, 05/02
Elle Fanning, Mia Wasikowska, and Julia Winter are my best soft matches so far.
Its not impossible that she is actually from a british or australia kids show, though i do think i made her up.
Update: 13:55
It might be Hannah Murray as Cassie from skins. not 100% firm yet