Re Two Thomas Grinyers beilieved to be from the same parents.
As you say, the most usual reason for the following to occur is that the child died in infancy and was replaced.
I have Thomas GRENIER son of Thomas (& Martha) b 1808 of Ovingdean, Brighton, Sussex.
Place of baptism: St Wulframs Church, Ovingdean, Brighton, Sussex.
Place of death: Coopers Plains, Brisbane
Spouse name: Mary PANNELL
Place of marriage: St. Nicholas Church, Brighton, Sussex. (IGI)
Date of marriage: 25 NOV 1832
And the other Thomas, also son of Thomas (& Martha) b 1811 of Ovingdean.
baptised St Wulframs Church, Ovingdean, Brighton, Sussex.
Place of death: 8 Richmond Place, Brighton, Sussex.(vol 2b page146)
Notes: 1841 Living at Borough St. Brighton.
Spouse name: Hannah BRYER
Place of marriage: Saint Nicholas, Brighton, Sussex (IGI)
Date of marriage: 28 JAN 1830
As both these Thomas' survived into adulthood, we firstly need to eliminate the possibility that there were two sets of Thomas & Martha Grinyer/Greniers residing in Ovingdean. Nothing on the IGI to that effect, anyone got a Sussex marriage index?
Lastly, I note both baptisms on the IGI are not not substantiated by a batch number that refers to the actual parish registers or Bishops Transcripts. You would therefore need to check out the actual parish registers or an authenticated transcript of them. What if one Thomas was illegitimate? If possible, that too needs to be checked out perhaps by searching for another adult Thomas in Ovingdean or something related to the support of base born children in the parish.
Roy G