Claravale, when was this? It really isn't possible to arrive at a sensible answer without knowing, for example, the date and place of birth of the one child, and of course the parents' names.
If it was far enough back, maybe it's just that the baptism records have not survived. Later on, maybe they belonged to the Free Church. Or they went to India and children were born there. Or Ireland, where birth registration didn't start till 1864 and church records are not so easy to get at. Or they tried Australia, New Zealand, Canada or USA, but didn't settle and came home.