I have learned not to trust the Census "relationships" column. I have people listed as "in laws" when they were in fact step parents, I have people listed as sister/brother/parent to head of house, when they are in fact sister/brother/parent in law. I have people down as "father" when in fact they are step father. In fact, I have learned not to trust the census at all and use it as a tool to go back to the original bmd record.
Philandlil, given that you have found this on the Census, it must be an 1851 or later Census. The children and particularly the younger child (the two year old) would therefore have been born well after civil registration came in (1837) , although it only became mandatory in 1875 so a few are missing. I would suggest you obtain a copy of the birth certificate of at least the younger child. As a start you can look on freebmd to see if you can find the reference. Failing that, buy a few credits for findmypast.com. You should then be able to find a potential birth entry and reference which you can use to order the cert from the GRO.