Meg is two months old in Reunion, and is the younger of the two.
Cornelia is engaged in Peggy, to be married that October. Assuming conception after marriage
and a full term birth, the earliest Terry could have been born is July the following year. That would make her just three during the reunion.
The absolute latest she could have been born is the August before Reunion, which would make her almost a year at that point - but having two kids nine months apart is pretty unlikely.
So she's probably born sometime between when the school opens in Switzerland in Barbara, and Genius.
Cornelia only gets brief mentions after her appearance in Reunion, mostly in the context of being the guardian of the Pertwee girls. There are no other kids mentioned. In Reunion she says that the kids are with their father and "Not that they are babies now,", which implies that she still has only two at that point. So she could fit in one more kid (or multiple) before the end of the series, but no more.