'Dans la poste" is specific - 'in'. "à la" is more general - 'at', which may mean 'in' or not. In usage you would only hear "dans la poste" when it was absolutely known that inside the post office was the location; otherwise it is "à la poste".

Türkçe düşünürsen olaya şöyle bak; dans türkçeye içindeyim diye çevrilir ve ilk cümlenin anlamı 
"ebeveynler postanenin içindedir" demektir  
ikinci cümle ise;
"ebeveynler postanededir" anlamına gelir (burada postanenin içinde olduğuna vurgu yapmamış)

Merci beaucoup !

Consider two situations.  You are at home, your parents left to go to the post office and will return shortly.  You are asked where are our parents.   à la poste might be appropriate. On the other hand if sitting on a bench outside the post office waiting for them to emerge and are asked the same thing, a reply: Ils sont dans la post might be more appropriate.   We learn from these artificial constructs of single sentences, when it might be better to tell a short story to clarify when one form might be right instead of another.