Presumably, if this young woman was Kate Bishop — or if the Russos were tweaking Marvel continuity and turning Clint’s daughter into the Marvel Cinematic Universe’s version of Kate Bishop — they would have cast a name actor for the part.
The only shot of Clint Barton and the mystery woman was from a distance, so it wasn’t entirely clear who he was with.
One of the more discussed moments in the Avengers: Endgame trailer was the appearance of Jeremy Renner’s Hawkeye — who’d been completely absent from Avengers: Infinity War — training a young woman in archery.
Avengers: Endgame Special Look: Every Easter Egg and Timeline Revealed