"My mother and my grandmother are at the beach in July."
Translation:Annem ve anneannem temmuzda kumsaldalar.
I don't think we'll add this, that's a very weird way to say it; which we could use in daily speech maybe, but it is weird to write like this unless you are writing a poem. But the "yüklem"(predicative) of this sentence is "kumsaldalar" and it should come at the end of the sentence.
hello :) i may be incorrect as i'm still learning but the things i find possible to make your sentence marked incorrect are : it may be because it's assuming that your mother is going with her mother (anneanne), it would be plajlarda to match the ending for they : onlar (-lar) and the -dir suffix makes it a statement of fact, and a bit like they are there the entire time and nowhere else. i hope that helps!