I would rather keep this translation. I don't agree that much with mukkapazza, to me "there is" better translation is "c'è".
This is true, but in "I have milk in the glass" does not make as much sense in English as "Ho latte nel bicchiere" does in Italian, which is why "there is" works here as well. Take as another example "ho una mosca nel piatto", you would have the same meaning if you used "there is a fly in my plate" in English. After all, this is the beginning of the Italian tree, I think it is worthwhile to introduce "avere qualcosa nel" also as "there is something in" so that you know you can actually use the sentence next time si pranza dalla nonna. "C'è" arrives later with the more complicated pronouns :)