Another option is to copy the value of the cell and paste it in the same respective cell, but with values only special paste option.
Input =NOW() in a cell.
Copy it Ctrl+C
Paste in the same cell Ctrl+Shift+V
In Google Sheets the checkbox itself is a data validation which means as soon as you change the data validation it will remove the checkbox. There is no way around this if you want to retain the ability to check and uncheck it manually.
If you want to have the ability to check and uncheck and prohibit checking column B unless column A is checked you will ...