The example that I've chosen is a recent purchase, rather than a portrait of a family member, and the subject is unfortunately not identified. I'll let the artistic merits of the cabnet card speak for themselves, but will add a little background to the costume that I think this child is wearing. The style of the Fauntleroy Suit was popularised as formal dress for middle-class American & British children in the 1890s and early 1900s, "much to the dismay of many young boys," after a book titled Little Lord Fauntleroy (online text, originally published as a serial in 1885) by American author Frances Hodgson Burnett, who also wrote that perennial children's favourite, The Secret Garden.
The style was derived from the illustrations of the clothes worn by the main character, which were pen-and-ink drawings carried out by Reginald Birch. These were based upon the "Van Dyke", an eighteenth century fancy dress which was, in turn, loosely modelled on Charles I court children's costume, and typified by Gainsborough's "Blue Boy."
Periodical brief revivals of the fad occurred after a film starring Mary Pickford was released in 1921, and again after an even better known version in 1936.
The photographer "Mac" was, in fact, the improbably named Algernon McIsaac who operated his Day & Electric Sudios at 212 King Street, Hammersmith, London in the early 1900s. McIsaac was born at Fareham, Hampshire (England) in early 1874, son of a pianoforte salesman, manufacturer and tuner, Thomas McIsaac (1825-1909) and his wife Emma (1838-1898; formerly Povey née Smith). He was working as a photographer by the time of the 1901 Census, but I have found no trade directory or similar entries showing his period of operation.
The only other example by "Mac" that I have been able to find is a carte de visite, also of a child, with a similar card mount design, shown above. I suspect that both of the portraits were taken in the early 1900s, perhaps between 1900 and 1905.
Post Script (5 September)
Well, it seems that I was wrong about this being a Fauntleroy suit! Birte Koch, who has a web site Album 1900, devoted to pictures of everday life over the period 187-1930, sent me an email pointing out that there were some significant differences:
Sorry to say but this boy is not wearing a fauntleroy suit. His suit reminds somewhat of a fancy musketeer-type suit. I add a picture of a typical Fauntleroy suit. Please note the cutaway-jacket (well, in my picture not that much cut away) (typically small, narrow and worn open because of the blouse) and the blouse showing lots of frills on front, cuffs and collar. And important is a sash. Fauntleroy suits often were done in velvet.