If a child cannot reach the upper third of the doorway and he is the primary occupant of said room, may the mezuzah be affixed lower than the upper third?
The Taz in YD 289 sk 3 is medayek (derives from a careful reading) that both the Rama and the Mechaber rule like the Rambam that if the mezuzah is placed below the upper third it is invalid even bediavad (after the fact). No one seems to mention any distinction based on the height of the room's occupants be they children or adults and it doesn't seem there is room to be lenient considering the adult, even without primarily living in that room, is still biblically obligated to have a mezuzah on that door.
There is a famous story about Rabbi Yaakov Kaminetzky, told in the book, Reb Yaakov, by Yonason Rosenblum, pp.326-327.
Reb Yaakov was particularly attuned to the dangers of exposing children to any kind of falsehood. He once visited the kindergarten of his son Binyamin's yeshiva and noticed that the mezuzah had been placed lower on the doorpost than halachically prescribed, so the children could reach it upon entering the classroom. The idea of getting children used to touching the mezuzah when they come into a room was a good one, said Reb Yaakov, but the means were wholly inappropriate. "Put the mezuzah on the upper third of the doorpost where it belongs," he said, "and let them use a stool to reach it. Otherwise they will grow up thinking a mezuzah can be put anywhere you wish. One does not raise children with falsehood."
I can't remember where I read this, but it was from a Chabad teacher.
It was asked if the Mezuzah could be made lower. And his answer was that rather than lowering the Miztvah to meet the child, get a step stool so the child can climb up to reach the mezuzah.
He compared this to the Cohen in the Temple who had to go up some steps in order to reach the Menorah and light the lamps.
I thought it was an important pedagogical lesson, even if it's not practical for every door in the house. Eventually the child will be tall enough to reach without the step stool though.
What I have seen done by a big Talmid Chochom was that they built a 3 foot high wall on two sides with a beam going across at the ceiling height. Then they put the Mezuza 2 feet off the ground where children were able to reach it.
2 mezuzahs may not be affixed to a doorpost. It is obviously not educational to do something against halacha plus kissing the mezuzah is a custom while the proper placement is halacha. A child will learn the prper placement and will grow up anticipating the time he can reach the mezuzah to kiss it.
I don't have any source, but I do have a suggestion. Hang the Mezzuzah in the appropriate place with a bracha, then hang one lower without a bracha or a klaf. In this way the child will practice kissing the Mezzuzah as it sounds like you want it to become a habit, without breaking Halacha.