“Like taking/stealing candy from a baby” means that something is easily done; a baby can’t fight back. Babies don’t have teeth to eat candy, of course, but the expression isn’t meant to be taken that literally. “Like taking candy from a baby” has been cited in print since at least 1896, with early citations coming from baseball and gambling.
 
The Len Barry hit song, 1-2-3 (1965), contains the lyric, “it’s easy…like takin’ candy from a baby.”
 
 
11 June 1896, The Enquirer-Sun (Columbus, GA), “A Dead Easy Mark,” pg. 4, col. 4:
He is tricky too, and made Mobile give up one run yesterday in the ninth inning that seemed like taking candy from a baby.
 
28 February 1897, Dallas (TX) Morning News, pg. 12, col. 2:
To take a kiss from this lovely maid would have been as easy as taking candy from a baby, but the timid boy was afraid to venture.
   
27 April 1898, Evening Post (Charleston, SC), pg. 7, col. 1:
Charleston Had an Easy Victory Over New Orleans Yesterday.
Charleston won from New Orleans yesterday. It was like stealing candy from a baby.
 
6 October 1900, New York (NY) Times, “The Variag’s Fast Speed; New Russian Cruiser Succeeds in Making 23 1/4 Knots,” pg. 2:
Ex-Sheriff William J. Buttling of Brooklyn has something like $4,000 outstanding in bets that the Republicans will be victorious this Fall in the State and Nation. Mr. Buttling is looked upon as one of the shrewdest bettors in Kings County. He has wagered, for one thing, that McKinley will carry that county.
 
“These bets have been forced upon me,” said the big ex-Sheriff, a day or two ago. “It makes me sad to have to bet this Fall. It’s like taking candy from a baby.”
 
19 October 1900, New York (NY) Times, “The Disinfection of Croker,” pg. 6:
But it would have been quite another thing to send actual money down to Wall Street to become the prey of the first bettor who had chosen to acquire it, and who would have felt in taking it, according to the picturesque expression of the Sheriff of Kings, that he was “taking candy from a baby.”