"I am where mom is."
Translation:Я там, де мама.
Literally: «I am there, where Mom [is]». It sounds unnatural in Ukrainian if you omit «там», it's used to join the two sentences.
This is a complex sentence made out of two parts (I am; Mom is), and 'there' and 'where' are used to join these parts into one. If you drop 'там', you'd end up with one part of the sentence being one word long, «Я». This sounds pretty unnatural.