Raise that uncertainty before the initial application, because the official FAQ says intended participating spouses and qualifying children should be included at the outset.
Make the unresolved choice visible
Ask counsel about the consequences of the actual options rather than assuming later joining is straightforward. Give each person’s circumstances and intended timing. A family may need further advice before proceeding, and a complete budget should not hide an undecided relative by quietly leaving them out.
Family questions come first in this sequence, not last. The official FAQ indicates that intended participating spouses and qualifying children should be included at the outset, which means the household conversation has to happen before the application is framed rather than after a fee is paid. List who intends to participate, who is undecided and who is definitely not coming, then put a date against each undecided person by which the household will decide.
Ask counsel what the actual consequences of each option are for this family, since assumptions about adding someone later should be checked rather than inherited from another programme. Hypothetical example: A feed milling family submits an application naming two of three children because the eldest has not decided, and the question they should have asked first, what including or omitting that child means, is still unanswered when the fee is paid.