Presumably you know something more about the family eg marriages of the children than you're letting on.
The family that keyboard found living in Yelling, a few miles from St Neots, in 1841 had a daughter Sarah who married James Watson in the Mar quarter 1851 (on GRO index as Knights) and was living in Hemingford Abbots Hunts in 1851, birthplace Yelling. The other family were living in Gt Raveley, five miles north of Huntingdon, so not exactly in the St Neots area, but both were born just outside St Neots. Much depends on the source of your belief that they were living in the St Neots area.