MIMF

My daughter recently got interested in this guitar building hobby of mine, and insisted on her own guitar:)
She's only 6 years old, but she's starting to get some of the chords on my old china-fender copy.
The neck is way too long for her, so I was wondering if I can build a 3/4 neck for the guitar.
I realize it would probably only have about 14 frets because of the neck pickup placement, but at least its nicer than playing with a capo.
As she gets bigger, I can just install the old full neck.
Do you guys think this could work?

Your scale length will change unless you move the bridge, and you'll have to calculate the fret placement for whatever scale you end up with but other than that it will work fine. The pickups won't be in the optimum place anymore, but it will work fine for what you describe

If you use a fretboard of the same scale length and cut off as many frets as you need to reach the desired scale length, the bridge will remain in the same location and the pickups will still be in the proper locations also. It simply means the neck will join the body at, as you say, at around the 13 or 14th fret, but that's not an issue. I say go for it! Cutting a 25.5" scale fretboard at the first fret gives you a 24" scale, for example. As a bonus, you can also use the fret slot for a zero fret...

And, try to find some much lighter weight wood for it. It's a temporary neck anyhow and won't have a lot of tension on it. Even pine has been used on(full scale length) telecasters, without truss rods.