George Richmond's painting of Swinburne and his sisters in childhood (click to enlarge and for more information).
Christopher Hollis's history of Eton contains the following description of Swinburne by his cousin Lord Redesdale:
What a fragile little creature he seemed as he stood there between his father and mother, with his wondering eyes fixed upon me! Under his arm he hugged Bowdler's Shakespeare, a very precious treasure, bound in brown leather, with, for a marker, a narrow slip of ribbon, blue I think, with a button of that most heathenish marqueterie called Tunbridge ware dangling from the end of it. He was strangely tiny. His limbs were small and delicate; and his sloping shoulders looked far too weak to carry his great head, the size of which was exag- gerated by the tousled mass of red hair standing almost at right angles to it. Hero-worshippers talk of his hair as having been a 'golden aureole'. At that time there was nothing golden about it. Red, violent, aggressive red it was, unmistakeable, unpoetical carrots.
His features were small and beautiful, chiselled as daintily as those of some Greek sculptor's masterpiece. His skin was very white — not unhealthy but a transparent tinted white, such as one sees in the petals of some roses. His face was the very replica of that of his dear mother, and she was one of the most refined and lovely of women. Another characteristic which Algernon inherited from his mother was the voice. All who knew him must remember that exquisitely soft voice with a rather sing-song intonation .. . . His language, even at that age, was beautiful, fanciful and richly varied. Altogether my recollection of him in those school days is that of a fascinating, most loveable little fellow. It is but a child's impression of a child. But I believe it to be just. [pp. 291-92; Hollis does not provide a source of Lord Redesdale's words.]
Hollis, Christopher. Eton: A History. London: Hollis and Carter, 1960.