August is mid-winter in Mauritius but it's close enough to the equator for the temperatures to still be quite warm. (Air temperatures will be roughly the same as the UK in August but sea temperatures will be higher).
Quote:
"The temperature on the coastal areas varies between 22�C in winter and 34�C in summer. The sea temperature varies between 22�C and 27�C. In the central part of the island, the maximum daytime temperature varies from about 19�C in August to about 26�C in February. The western and northern regions are warmer and relatively drier than the East and the South."
Source:
http://www.mauritius.net/general_info/index.ph p
(That's the official Mauritius tourism website. Browse around for further information).
While I am sure that you could find something to keep an 8-year-old occupied, I wouldn't really regard Mauritius as an ideal destination for a child of that age. The flights are around 12 hours and likely to be stressful for everyone concerned!
Save money and head for somewhere closer!
Chris