For a family with 3 kids 14-18, how much time should be allocated for a visit to the VDNKH amusement park, and what attractions are must-see?
Allocate at least an hour. Must-see is walking up the main alley, seeing all the fountains and pavillions. Then you can take some side detours but that's optional.
If you want to visit anything, my recommendation is space museum that's outside of ВДНХ, under the rocket monument.
You can also spend half and hour to see the iconic "Рабочий и Колхозница" monument in the walking distance outside of main ВДНХ entrance.
Then you can leave via monorail if it still operates.
Thus a good complex visit may take around three hours. I'm pretty sure you won't run out of things to see.