Children under the age of 15 are not charged a fine for overstaying but still get overstay stamps when they leave the country. Not 100% sure of any future issues, but probably none.
Just don't visit the same countries all the time and get repeated SETVs from the same place. She may have a problem doing it for 2 years as that is probably 6 or 7 in 2 years, depending how long her holidays outside are. Just see how it goes. METVs would make a lot more sense for that length of time.