Hi,
had a look at the actual page 1851 Isaac is given as 30 which would give him a birth year c 1821.
Lindfield very feint 1841 but have found the family.
HO107/1114/3 FOLIO 10 PAGE 12 LINDFIELD
THOMAS CHILDS 50
ELIZABETH 50
JOHN 20
ISAAC? 15
JAMES 12
THOMAS 9
1841, ages over fifteen were quite often rounded down to the nearest five or ten.
www.familysearch.org have christenings for this family Horsted Keynes and there is a marriage West Hoathly 28/11/1810 Thomas Child of Horsted Keynes to Elizabeth Atherall of West Hoathly.
Hope this helps.
Best Wishes
Diane