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Originally and chiefly American English, the colloquial phrase new kid in town designates a newcomer, a recently arrived person.
—Synonym: new kid on the block.

However (apart from two texts in which the context is insufficient to determine the meaning of the phrase—cf., below, quotations 3 & 7), all the texts containing the earliest occurrences of new kid in town that I have found are birth announcements. But this is perhaps a humorous use of the phrase: the fact that, in these texts, quotation marks often enclose (there’s) a new kid in town may indicate that this particular sense (a newborn) is a jocular reference to the broader sense (a newcomer).

These early occurrences of the phrase new kid in town are as follows, in chronological order:

1-: From The Western Spirit (Paola, Kansas, USA) of Friday 30^th November 1883 [page 3, column 4]:

Tʜᴇʀᴇ’ꜱ a new “kid” in town
He came the other day,
And he’s a lah-da-dah
Ed. George’s folks all say.
(Weight 10 pounds.)

2-: From one of the unconnected paragraphs making up the column Town Talk. Brief Mention of News Caught Here and There by the Journal Corps, published in The Journal (Evansville, Indiana, USA) of Wednesday 2^nd February 1887 [page 5, column 3]:

“There’s a new kid in town,” and officer Litty says he will be ready to go on the force in just twenty-one years.

3-: From one of the unconnected paragraphs making up the column Courier Cullings, published in The Colwich Courier (Colwich, Kansas, USA) of Saturday 24^th December 1887 [page 5, column 2]—however, here, the context is insufficient to determine the meaning of the phrase:

There’s a new kid in town. Sh-h-h-h!

4-: From Hayward Happenings, dated Hayward, Minnesota, Monday 8^th July 1889, by ‘Honest Johnnie’, published in The Albert Lea Enterprise (Albert Lea, Minnesota, USA) of Thursday 11^th July 1889 [page 4, column 2]:

Another new kid in town. It made its appearance in the family of Mr. and Mrs. Ed. Johnson.

5-: From one of the unconnected paragraphs making up the column City News Briefly Told, published in the Daily Morning Patriot (Harrisburg, Pennsylvania, USA) of Friday 11^th October 1889 [page 1, column 7]:

Hats off to Officer Brownewell, “there’s a new kid in town.”

6-: From The Eureka Herald (Eureka, Kansas, USA) of Friday 14^th February 1890 [page 4, column 2]:

Alec Welsh reported “a new kid in town,” Monday morning—born to Bassett’s Mr. and Mrs. William Goat.

7-: From Hayward Happenings, dated Hayward, Minnesota, Monday 30^th May 1892, by ‘Boot Jack’, published in The Albert Lea Enterprise (Albert Lea, Minnesota, USA) of Thursday 2^nd June 1892 [page 2, column 3]—however, here, the context is insufficient to determine the meaning of the phrase—bran new is a variant of brand new:

There are two bran new kids in this town. What’s the matter with Hayward?

8-: From The Farmers’ Vindicator (Valley Falls, Kansas, USA) of Saturday 23^rd September 1893 [page 2, column 5]:
—This paragraph alludes to the then President of the USA Grover Cleveland (1837-1908), whose youngest daughter, Esther, was born on Saturday 9^th September 1893; his eldest daughter, Ruth, was born in 1891:

There is a “bran new” kid in town. Grover is two daddies now and baby Ruth has a little sister to play with. This same thing happened about 5,000 times in this country but not in the white house.

9-: From The Anaconda Standard (Anaconda, Montana, USA) of Saturday 5^th February 1898 [page 9, column 2]:

New Kid in Town.

Great Falls, Feb. 4.—Born, to Mr. and Mrs. Henry Tietjen, a 10-pound boy.

10-: From the Daily Democrat-Messenger (Missoula, Montana, USA) of Thursday 7^th December 1899 [page 1, column 2]:

THERE’S A NEW KID IN TOWN.
Sam Tom Is Hitting the High Places Around Chinatown Today.

There’s joy in Chinatown today, and the almond-eyed residents of the district have set the town on edge and are butting holes through it, all on account of the advent of a chubby little Mongol in the home of Sam Tom, Missoula’s popular Celestial. The kid, who by the way, will be a voter when he becomes of age, arrived at an early hour this morning, and is one of a coterie of five children, four boys and one girl, all of whom were born in Missoula.

The earliest occurrence that I have found of the phrase new kid in town used in the sense of a newcomer is from “Chaw Raw Beef”—A Story of the Ol’ Swimmin’ Hole, a children’s short story published in The Daily Telegram (Long Beach, California, USA) of Saturday 30^th August 1913 [page 13, column 5]:

Roland Wilberson had not been in Cherrydale ten days before the boys set him down as a “sissy.” […]
[…]
[…] And there, hanging on to a branch with one hand, the other swinging free with the rope he was going to hurl to Bud, hung the “new kid in town, the sissy.”