Translation:The grandmother buys trousers for her grandson.
Yes, it is used a lot. His/her has really 2 meanings in English. Many languages like Russian for example are more precise. Esperanto too doesn't have that ambiguity that English has. A sentence like La patrino amas siajn infanojn is a very common sentence. You can't really avoid "sia".
I think they can't account for every reasonable translation straight away, which is the reason for the report button and sadly American English takes precedence here as it does on a lot of the web (I'm reminded of the joke about the Catholics being behind a brick wall in heaven with the explanation that, 'They think they're the only ones here').
Although, the Welsh course does seem to favour rest-of-the-world English, which is one plus point about it.