"I am there where mom is."
Translation:Я там, де мама.
I think this sentence sounds less natural with «є», because «є» is only used when you want to emphasise existence. «Я там, де є мама» (and Russian «Я там, где есть мама») sounds as if your mother is not a single person, only one, but as if you might have more than one mother. I.e. you're emphasising not the location of the only mother, but existence of mother at some place. At least that's how I'd understand this sentence.
Of course, this might work because a child might have more than one person they call mother (e.g. mother, step mother, nurse), but this sounds less natural to me.