I believe the children above were:
Eleanor Fellowes (daughter of Diana's sister Jane)
Prince Harry
Alexander Fellowes (Eleanor's older brother)
Emily McCorquindale (daughter of Diana's sister Sarah).
This seems to be Regency period costume- on the second page of the Charles Spencer and Victoria Lockwood wedding thread on this site
www.theroyalforums.com/forums/showthread.php?t=4098&highlight=Victoria+Lockwood
one of the members posts that "the outfits were inspired by two paintings hanging at Althorp House. The pageboys outfit was inspired by a Joshua Reynolds 1779 portrait of Lord Althorp."
I hope that helps as well!
Eliza