Hi TK2562,
There is no uncertainty here.
480 Chapter 10 definitively states the expense only extends to a spouse.
This is supported by
Tax rules on other types of travel and related expenses (490: Chapter 8)
There is no mention in either of those 2 pieces of guidance that states it can be extended to a child or other family member.
The customer is only entitled to claim travel expenses for the spouse if they are accompanying them for practical purposes directly associated with the purpose of the trip.
If they are just accompanying them so as not to be separated, they cannot claim the expense.
This would be so for a child or other family member accompanying them on the trip.
There are special rules for when the employee travels abroad for more than 60 days at a time where a child can visit up to twice during that period.
The rules can be found at
Special tax rules on foreign travel (490: Chapter 7),
Please see section 7.6
But that does not appear to apply here.
Thank you.