Logic Masters Deutschland e.V.

Normal sudoku rules apply. Place the digits 1-4 once each in every row, column, and 2x2 box.

67: one cell in the grid is the six-seven cell — it is up to you to find which. Its placed digit (1-4, for its row, column, and box) is normal, but for value clues (cages, X/V pairs, dots, and the like) its value is both 6 and 7: each such clue reads it as a 6 or as a 7, whichever it needs, and across the puzzle at least one clue reads it as a 6 and at least one reads it as a 7. For example, a cage might add this cell as a 6 while an X pairs it as a 7.

Killer: digits inside a caged region must not repeat and sum to the total shown in the corner.

Thermometer: digits must strictly increase along each thermometer, starting from the bulb.

Cells connected by an X sum to 10. Not all Xs are given.

Digits may not repeat along marked diagonals.

Solve in VariantDoku (iOS) or on SudokuPad.

Postscript. Happy Children's Day! Although, to be fair, 67 is probably not only for children.