After the first visit, she will likely ditch the heels. Let her do what she wants – BUT I’d tell her if she wears the heels down walking in them, she buys the next pair or pays for the repair.
Flats. Now, if it’s a tiny heel, maybe, but the tours generally involve walking all over campus, up and down hills and stairs.
The only students she’s likely to see are tour guides. Remind her she will be interviewing with admissions people, who are not likely to be impressed by heels on a 13 year old.