The BVRI shows the following children of Robert and Elizabeth:
William born 15 April 1806, christened 21 Sept 1806
Susanna born 21 May 1808 chr 9 Oct 1808
Sarah born 24 March 1810 chr 14 Apr 1811
then none of the others in the 1841 census are shown as christened in Melbourn.
I wonder if they may have joined a non conformist church as there's a burial on the burial index at
http://www.cfhs.org.uk/Search.htmlof Robert King at Melbourn Independent
Hopefully someone will have access to Boyds Marriage Index which may show the marriage in 1806 or before
The 1817 marriage seems a bit too late and there's a baptism to Robert and Elizabeth of Comberton at Papworth St Agnes in 1818 on the baptism index on the same Cambs FHS site mentioned above
Regards
David