I believe the children above were:
Eleanor Fellowes (daughter of Diana's sister Jane)
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
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!