"Excuse me, is the bathroom here?"
That would be “Excuse me, is here in/at the bathroom?“
在 is (in this case) a verb meaning “to be (located) at/in (a place)”, so 在洗手间 “to be in the bathroom”. Whereas 洗手间在这里 “the bathroom is here/at this place” (literally “at here”, but English doesn’t use a preposition with “here/there/where”). Add 吗 and you get the corresponding yes-no question: “Is the bathroom here?”
An alternative would be 这里是洗手间吗 “is this the bathroom”. Here you have to use 是 instead of 在 because you’re not asking where “here” is located. You’re basically asking “this place = bathroom?” That being said, this way of asking sounds to me like both you and the addressee are already in the bathroom when you’re asking the question (it’s the kind of formulation you’d use when you’re unsure of where you are, a bit like English “what is this place?”), whereas with 洗手间在这里吗 you’re probably standing outside the door pointing at it.
"Excuse me, here is the bathroom?" Would be the literal translation of your answer. That would be more of confirming that you are at the bathroom, rather than asking where it is in the first placr.