My Parents want to go See the Cherry Blossoms in DC?

They did a study not too long ago which figured out how many psychopaths live in different parts of the country and Washington DC came out on top. I think you’re more in danger of that  

It's strongly discouraged, and Metro has closed down the stations near the blossoms.

You have one viable option:  to drive around the area, keeping in your car.  You can give them an idea of what they look like.  Look at a google map of DC and you can see some options.

If they stay away from other people, they could do that. However, if they are in poor health and/or older, they shouldn't take the risk. They might check to see if the National Park Service webcam will be operating this year. It's not quite the same as being there but completely safe.

I wanted to go see the sandhill crane migration this week which would not require me to be in crowds but decided not to go even though it would be in my own car because I have no way of knowing how many other people will be around. The migration is also covered on webcams that I can watch every morning/evening. 

I think if you're going to be walking around outside to see the cherry blossoms then it will be fine.  You will put yourself at risk if you are entering large crowds, particularly indoors.  Since the other major DC attractions are closed, you probably won't be able to do anything but walk around outside anyway.

Well, there likely aren't too many other people going to see them at the moment, but staying away from any potentially large gatherings is a good idea.  You don't know how many other people might be there.