This is the type of thing that will vary with different countries and localities. However I would have no problem with this being called a park, especially if it has some paths and trees. It's a smallish park but being about the size of a football field the locals might very well call it "the park".
It could also be called a square, particularly if it's roughly rectangular. A square will often have a significant area of open ground but there's no reason for it not to have a few trees dotted around.
People living in the area might refer to it as "the gardens" or something similar, especially if the flower beds etc are a prominent feature.
In Aust/NZ it is certainly not a yard. A yard is either a commercial property like a timber yard or a car yard, or part of a private property adjacent to a house (front yard, back yard).
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