Either simple past or present perfect works here - it depends on what nuance you want to impart.
So, it depends. If the topic is something which, by the terms in the sentence, occurred in the past and has little importance to the present, then simple past is called for.
If there were an indicator in the sentence of a past event, then you have to use simple past: "Yesterday, Mom bought some candy for us". If there is no indicator, then "Mom bought some candy for us" means at some point in the past, Mom bought candy - even if it were just a few seconds ago.
If the past action has present relevance, then present perfect is called for: "Look, Mom has bought candy for us" - something said as Mom walks towards us in the present with a bag of candy.
In American English, the distinction is not always strict: "Mom has just bought candy for us" mean the same thing as "Mom just bought candy for us", although this latter sentence may be an example of a colloquial omission of the auxiliary "has", but that point could be debated until the end of the universe.