They are obviously still alive as they reappear in 1861.Could they have been sent to stay with relatives prior to the birth of Daniel?
Using a wildcard search the only Rosanna of the right age in 1851 is this one. The names are just dittoed in the original and the fact that the servant has the same name makes me wonder if it was just careless transcribing.
HO107/2003/281/30
Newport Lane, Burslem, Stoke Upon Trent
Venables William 57 Head Beer Seller Burslem, Staffordshire
Venables Martha 53 Wife Burslem, Staffordshire
Venables Bagnall 20 Son Potter Presser Burslem, Staffordshire
Venables William 16 Son Potter Presser Burslem, Staffordshire
Venables Rosanna 6 Niece Scholar Burslem, Staffordshire
Venables Mary 16 Servant Domestic Servant Burslem, Staffordshire