I don't know if that is how the comma is used in Ukrainian, but in English the comma would not be used here unless you wanted to ask where Mom is currently located even though there is no question mark at the end of the sentence. Either remove the comma or replace the period with a question mark. Remember, Punctuation saves lives. Let's eat Grandma vs. Let's eat, Grandma.
The comma in the sentence implies "I am there, where is mum?"
It doesn't, the comma in this sentence is a matter of your choice of style, but it clearly means "I am where my mum is" and not "I am there, where is my mum?", whether the comma is there or not.
_I am there, where is my mum? _
could be translated as:
Я там. Де мама?
Я там, а де мама?
Я там, а мама де?