I think a 20" is easer to learn on. You could get one as a starter unicycle (and a future loaner) for her to learn with. Once she gets the basics down, transitioning to a 24" is easy. Can you borrow a 20" for a week?
Scott
I started on a 24 but i was getting frustrated & I was terrified of getting hurt in a UPD (UnPlanned Dismount). So I got a 16, it gave a big boost to my confidence. Then I got a free 20 and rode that before going back to the 24.
I have some paralysis in my legs (need AFO braces to walk, see avatar) so everything took longer. If I could learn, ANYBODY can learn.
Timeline, I averaged 1 hr/day, 5x a week.
Could ride the 16" 150 ft @ 2 months
Moved from 16" to 20 @ four months
And back to the 24 @ 6 months
Rode off my first curb @ 8 mo.
Did my first Muni @ one year.
Road SIF, SIB, & backwards @ 2.5 yrs
Each wheel took me about 30 min to adjust to the basics, and after I was comfortable on each it could take me up to 5 min to re-adjust to one of the other sizes.
I still can’t hop well enough to be any use, or free mount. 
What is the cranks length?
I learned on 26" with 145 cranks - perfect.
May be you should try longer cranks? 
Cranks…
Uhhh…I have no idea what size the cranks are. 
It’s a stock 24" Torker LX.
-JN
There are 150mm cranks. Must be good enough. Just keep on trying!!!
If interesting - here is my story of learning, may be some tips will be usefull.
Do not quit! 
That seems very long for a 24" wheel. I expect the OP’s daughter would do better with something a bit shorter.
150’s are pretty much standard for a 24" (at least here in North America) and should be ok for learning on. If they get much shorter then there could be a control issue which might not be the best plan for someone learning to ride.
Shorter cranks for me have always been associated with less control until I get used to them.
The OP mentioned that his daughter was able to ride “a smaller one” much more easily. Of course we don’t know the wheel size or the crank length, but I’d imagine a 20" with 114mm cranks. If she’s got on well with 114mm cranks in the past then moving up to 150mm cranks will make an enormous difference to the feel of the unicycle - probably a bigger difference than the change from a 20" to a 24" wheel. I honestly think shorter cranks would be worth a try.
Note: I’ve had a quick look at the 24" yikes at unicycle.uk.com and while the Munis have long cranks, the beginner unicycles have 127mm cranks, e.g. http://www.unicycle.uk.com/unicycles/freestyle/24-club-freestyle-unicycle-black.html
Gonna try some new cranks
It’s a “spec” 24" Torker LX, and from what I see online, it looks like they come with 150mm cranks. Maybe I’ll order her up some shorter cranks and surprise her with them when I take her uni down to her next month. 
I’ll have to make sure to get the right tools for her to change them out later if she wants.
Thanks.
-JN