For each position write down which numbers are still possible.
Clues about a single position ("greater than 6") cross out many numbers there.
Clues about all positions together ("all the numbers make 12") fit at the end - use them to check which options are left.
Example
For parents
What does this exercise practice?
The number lock links properties of numbers (even, greater, digit sum) with systematic elimination.
Common pitfalls
"Greater than 6" also rules out the 6 itself. Counting the boundary in often produces two answers instead of one.
How to practice at home
Take a real combination lock from a bike and give each other clues about the code you set.