The earliest brooch I seem to have is from a 1933 photo where it appears that QEII is wearing a small brooch on her beret
On some other occasions what I think may be a brooch is probably a dress decoration.
From 1939 it is clear that both The Queen and Princess Margaret are wearing brooches on various occasions.
Here is one of QEII in 1939
Christie's catalogue of Princess Margaret's jewels states that George VI gave the brooch below to her when she was 8 so it would seem that both princesses were wearing brooches from a young age.