I exhausted my search for a 4 foot, 16" wheel giraffe unicycle, for my daughters birthday. do you think if I got a 20" wheel 5 foot unicycle from torker and cut the seat post down to fit her, it might work? I can get one of those brand new shipped for under 150.00. then cut the top tube down to fit her. any thoughts? I guess I would need 100mm crank arms too, like shes use to.
Height constraints on a giraffe should be less of a problem than with a regular unicycle.
That being said, it is likely that you will have to modify the frame as well as cutting down the seatpost. Cut the frame down to height, then re drill and cut a slot at the top. Theoretically you could fit a very young child on a giraffe if you wanted to. From a picture of the uni, it looks to have an integrated seatpost clamp, so a replacement clamp would be needed.
100mm cranks are going to be difficult to source, you will likely need to order them from someone who sells tandem bikes as child stoker cranks.
The smaller the rider, the bigger the unicycle. So for a 7-year old, a 5’ giraffe is like riding something taller for a bigger person. What this basically means is that you want to make sure she’s ready before cutting her loose on one.
IMHO, to best prepare a kid (or adult) for giraffe riding, they should be comfortable with idling and riding backward. Each of these skills can be essential if the rider gets into a situation where it’s impossible to ride out going forward. If you can’t idle or back up, you may be forced to dismount ungracefully, possibly into whatever it is that’s blocking you.
Also, it’s important to feel comfortable making controlled/planned dismounts in any direction. That is, to any point of the compass. Sometimes you end up falling to the side. If this induces panic, injury may follow. If you’ve practiced doing it before, you will know there’s nothing to worry about; just got to make sure your feet have a safe path to get under you. Do a bunch of dismounts in purpose so you know how to do it in each direction.
Especially with a small child; learn those skills first, and you will be pretty safe up there. If you can only ride forward, you can still ride a giraffe, but you shouldn’t ever ride near anything. 
I learned at 17, on a Schwinn Giraffe 6’. I think most of my idling was learned on the giraffe, though the backwards was surely learned on a lower unicycle. 
In a simple unicycle there are two limits to the size of the wheel than can be ridden.
The instep length is the obvious one but also important is the consideration of the strength to accelerate a large wheel with a lot of inertia fast enough for optimum balance performance.
The giraffe configuration somewhat overcomes both of these limitations for small riders in different ways. A child could even straddle a 36er giraffe because the crank-axle to seat distance is arbitrary in a giraffe.
However they would struggle providing enough torque to get that big wheel moving so of course you wouldn’t use such a huge wheel. However, even a 24 is not ridiculous if you compensate the torque limitations.
FIrstly the giraffe allows us to use a step down ratio on the chain drive. A 24 inch wheel can be made to feel quite similar to a 16 with 3:2 step down ratio on the chain drive.
The bigger wheel provides the benefits of a lower rolling resistance while the ratio makes it feel small. This may be an important advantage to a small rider.
However it would still be essential to minimise the inertial mass of the wheel. A twenty inch wheel with a light weight light tyre is one possibility. The tyre is the most important consideration because it the largest mass in the wheel and its mass is at the absolute maximum radius of rotation.
Personally I would try a 24 inch which allows a very light tyre like a Maxxis DTH. This tyre only weighs only 375 grams. AFAIK it isn’t available in a 20 inch or i owuld suggest that. I use a DTH on my 24 inch and it is far, far more responsive than my 20 inch with a 700 gram (Maxxis Hookworm) tyre.
I expect that with the 3:2 ratio the 24/DTH combination would not feel so different from a 16 inch with a slightly heavier tyre.
Hpefully up can cut the frame and post to suit.
Just cut the frame. The post will still fit at 300mm. I cut my Torkers down for my 4 and 5yr old and has been amazing!!
I guess it worked just fine
An hour or two after getting her giraffe unicycle, look at her go. She never rode one before. I’m so proud of her. I just cut 4" from the seat post tube, and welded it back together. didn’t change anything else.
Excellent!