I can't see them in 1901 either. The younger children seem to be registered as Allum, but I can't see births registered for the older children or father Charles
I keep coming back to this family in 1891 living in Fullers Hole, Norwich. Apart from Charles the other majority of the children were born in Norwich, Norfolk. By 1901 the family - minus Charles - were lodging in Suffolk with one young child listed as born in Hastings, Sussex. I can't see a birth reg for this Charles, but there is a tree online with a birth cert for sibling Frederick born 19 Oct 1881 in Knodishall, Suffolk which lists his father Charles as a Marine Store Dealer
Charles Allum 43 Hawker b Ipswich, Suffolk,
Sophia Allum 33 Hawker b Suffolk
Charles Allum 17 Hawker b Cambridge
William Allum 11
Frederick Allum 9 Suffolk
Alfred Allum 4
Arthur Allum 6 months
G12 Piece 1518 Folio 71 Page 13
Kay