I assume that the freeze amendment did not also give 100% vesting. If it did, no brainer, since you can't undo that by amendment.
My judgement is that the two non-family EEs who terminated do constitute a partial termination. but there is still a facts and circumstances issue in this determination. For example, if one of them terminated by dying, then you probably don't have to count that against the 20% test.
Look at IRS form 5310 and instructions. There is a question (don't remember the line number but it is the bottom of page 2) that asks for the number of non-vested terminations by year. If over 20%, you are requested to show why it will not constitute a Partial termination.
The fact that the son and wife quit (by the way, 4 out of 6 employees quitting is 66.7%, not 60%) should not have any effect on the vesting of the 2 others. If the 2 others are a partial termination (2 out of 6 is 33%) then that by itself will cause 100% vesting.