The final arbiter of what is allowed, or not allowed, for a given mortgage is the mortgage provider. If you were to omit essential information during your application, and if the mortgage provider were to find out, the consequences could be serious.
Bearing in mind I'm neither a lawyer nor a financial expert, the full definition of what the Financial Conduct Authority defines as a family member may be of interest.
PERG (Perimeter Guidance Manual), section 4.4.9, says:
'Related person' is defined in article 61(4)(c) of the Regulated Activities Order as meaning the borrower's spouse, civil partner, parents, grandparents, siblings, children and grandchildren. An unmarried partner of the borrower whose relationship with the borrower has the characteristics of the relationship between a husband and wife is also included; this can include a person of the same sex as the borrower. Stepchildren, however, would seem to be excluded.
However, in the previous paragraph, it says that this is only relevant if:
the lessee is a 'related person' to the borrower
As the question states, a child is too young to be a lessee (i.e. tenant).
However, the aforementioned Regulated Activities Order states that this is relevant if the property:
is used [...] as or in connection with a dwelling by the borrower [...] or by a related person;
suggesting that mere use by a relative is sufficient to meet the definition.
(See also here, here, and here).