That would require the letter "ר", which isn't there.
Also if you want your transliteration to indicate pronunciation, you'll want to use something other than just the letter "h" to represent the Hebrew letter "ח"; e.g., "ch" and "kh" are often used in an English-based setting such as this course.
(This comment is for any reader who's taking Plasticcaz1 seriously.)
This is one of the areas in which this course falls short: they do not teach nikud, and although they use it sometimes, they don't always do so where it's needed, as here. This course is managed very inconsistently, and DL itself is poorly organized, which are both reasons to use additional resources beyond this course to learn Hebrew.
I think it would depend on the context really.
Is the whole sentence "Mom, the one in the kitchen"? That's really just a re-phrasing of "Mom who is in the kitchen".
Or, "That's my mom in the kitchen". That's re-phrasing of "That's my mom who is in the kitchen". Either way, the sense is still there.
In that case, the "in the kitchen" phrase is separate from the object of the sentence: "mom". It is as adverbial phrase modifying WHERE we will surprise mom. I think in that context that part of the sentence is identical. I'm not native, but it's probably something like: "בואו נפתיע אמא במטבח"
If I understand correctly your question, you are asking about the patach (sometimes spelled pataḥ) vowel. There is a distinction in pronunciation and also in meaning. Without the vowel indicated, it could be a shewa (sounds like a short e) instead of patach (which sounds like an a). The difference in meaning is that the patach indicates "in the" while the shewa "in a." Intro grammars to biblical Hebrew will explain the differences between types of shewa and if you google "Hebrew shewa" you will get plenty of explanations. According to Bob Dylan, however, mom's not actually in the kitchen: "Mama's in the factory, she ain't got no shoes / Daddy's in the alley, he's lookin for food / I'm in the kitchen with the tombstone blues."
What exactly is "mummy"? What a small child would call her own mother? Most probably, the neutral word אמא. Probably these days you can hear אימוש /imush/, applying the affectionate suffix /ush/ (borrowed from Russian AFAIK) which is on the rise now in Israel. But it's likely to fade away in a couple years.
If you mean "mummy" as in "a preserved dead body", מומיה.
Mama's in the kitchen should be okay too. I do the Duolingo stuff with other languages, and they are much better with respect to the English. I am a native English speaker and been marked wrong because of variations that differ from what the people who put this program together think the English ought to be. Quite frequently, by the way, they are wrong, but that's another story.