The Internal Revenue Service allows for a child deduction on personal income taxes. A popular joke is:

“Children may be deductible, but they are still taxing.”

“Children are deductible—but they can also be very taxing” has been cited in print since at least 1960, when the joke was included (without credit) in an Earl Wilson syndicated newspaper entertainment column.


IRS
Child and Dependent Care Credit at a Glance
A credit for the costs of care for a qualifying individual to allow you to work or look for work. The dollar limit on the amount of the expenses you can use to figure the credit is $3,000 for the care of one qualifying individual or $6,000 for two or more qualifying individuals. The amount of your credit is between 20 and 35 percent of your allowable expenses. The percentage you use depends on the amount of your adjusted gross income. 

Google News Archive
17 May 1960, Beaver County (PA) Times, Earl Wilson syndicated entertainment column, pg. 13, col. 4:
EARL’S PEARLS: Children are deductible—but they can also be very taxing.

Google Books
2000 One Liners
By Larry Oliver
New York, NY: Pocket Books
1970, ©1969
Pg. 63:
Although children are deductible, they can also be very taxing.

17 June 1971, Boston (MA) Herald Traveler, “Hub-Bub” by J. J. Smith, pg. 7, col. 7:
Publicity director Sy Evans of General Cinema Corp.: “Although children may be deductible, they can be very taxing!”

Google News Archive
17 April 1972, Florence Times/Tri-Cities Daily (Florence, AL), “The Light Touch” by Tom Jones (Tom Jones Insurance), pg. 5, col. 7 ad:
Although children are deductible, they can a be very taxing.

Google Books
14,000 Quips & Quotes:
For Speakers, Writers, Editors, Preachers, and Teachers
By E. C. McKenzie
Grand Rapids, MI: Baker Book House
1990, ©1980
Pg. 70:
Children may be deductible, but they’re still taxing.

Google Books
King’s Treasury of Dynamic Humor
By King Duncan
Knoxville, TN: Seven Worlds Corp.
1990
Pg. 47:
Even though children are deductible, they can also be very taxing.

Twitter
Terrell Lewis
‏@SgBz
Even though children are deductible, they can also be very taxing.
7:28 PM - 18 Jul 2009

Twitter
Brian Coplin
‏@BrianINC
Children may be deductible, but they are still taxing.
1:46 PM - 12 Dec 2009

Google Books
The Mammoth Book of One-Liners
By Geoff Tibballs
London: Constable & Robinson Ltd.
2012
Pg. ?:
Children may be deductible, but they’re still taxing.