Bikes are designed to fit with some seat extension, traditionally a handfull of exposed post. Some styles of bikes (compact frame) are designed to be used with about 6-8" of post showing.
With the 24" bike, how long is the seatpost.
How much extension does your daughter need right now? Is the saddle correctly positioned (ie not too low)
That will give you an idea of the growth potential.
The other important dimension is reach from saddle to bars. Is the 26" bike too long?
Generally, if your bike has no exposed seatpost it is too big. This means the reach is also too long.
Is there any difference in weight or rideability (which does she prefer?)

The advice for adults who are inbetween sizes is to opt for the smaller bike.
12" is very small for a 26", I have never seen below 13". The Kona kids bike uses 12" frame with 24"wheels and looks nicely proportioned.