In English, the word "yell" is often associated with angry feelings. If the woman was calling her son in an angry voice, I would say, "she yelled at her son to get her medicine".
You could also use the word, shout, which suggests louder volume and authority. For example, "she shouted at her son to get her medicine".
Another suggestion is, "she cried out to her son". This suggests more desperation and fear in her voice.
For example, "she cried out to her son to get her medicine".
Hope this helps