Here is what I found in a PDF from the Congressional Research Service with regard to Secret Service Protection:
http://fas.org/sgp/crs/homesec/RL34603.pdf
I wish they would define “immediate families”
President, Vice President, President- and Vice President-elect
• the immediate families of those listed above
• former Presidents and their spouses
• former Presidents’ children under the age of 16
• visiting heads of foreign states or governments
• distinguished foreign visitors and official United States representatives on special missions abroad
• major presidential and vice presidential candidates, within 120 days of the general presidential elections, their spouses
• former Vice Presidents, their spouses, and their children under the age of 16
>>• major presidential and vice presidential candidates, within 120 days of the general presidential elections, their spouses
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It doesn’t list “children” at all for major presidential and vice presidential candidates.
Maybe it’s changed since the publication date of the document, which is June 14, 2014.