First of all, the second sentence requires a minor correction. The article should come before the adjective:
？ This movie is a good family entertainment.
But, with that correction, it would still be unusual.
It would be just as unusual as the following:
？ These movies are two good family entertainments.
Any use of the count version in such a sentence is unusual enough to probably be corrected by someone.
To use an article in this sense would require inserting something else that's being counted instead:
✔ This movie is a good example of family entertainment.
✔ These movies are two good examples of family entertainment.
If you look at an actual example of a family entertainment that Google Ngram Viewer points to, you see that it's not being used as a noun but as part of an adjectival phrase (emphasis mine):
An example of a family entertainment attraction at a shopping mall today is Nickelodeon Universe at the Mall of America in Minneapolis …
It works in that context, because what's being counted is attraction, not entertainment.
The Wind in the Willows: A Family Entertainment
Here, it's part of the title of a book. Titles and heading often don't follow normal rules of grammar.
Note that this isn't actually ungrammatical. But in other contexts, it would certainly be unusual.