Hi Trish
Think I could have found them and it would appear to be a change of name. The address in 1901 is 11 Preston Road, West Ham, Stratford, Essex and so I checked this address on the 1891 census and there they were (I think) with 2 older children also:
Wm. Weaver 40 Metal Maker b. Whitechapel
Sarah Weaver 36 Metal Maker Box Maker b. Bethnal Green
Wm. J. Weaver 17 b. Bethnal Green
Sarah Weaver 14 Metal Maker Box Maker ditto
Josh. Weaver 12 Errand Boy ditto
Mary Weaver 5 b. Stratford, Essex
Harry Weaver 3 ditto
Ref. RG12 - 1310 - 83 - 32
Annette