Do you want to show how the plant grows, or how a flower opens up? As you know, it goes pretty slowly. Then try a so-called TimeLapse app that makes the process seem much faster.
Three apps to try are LapseIT, Osnap! eller iMotion.
The last one can also create stop-motion movies, i.e. put together individual pictures so that it becomes like a movie.
Tip: Try to put your iPad in the same place every day in a window and take a picture. If you put it together you get a picture of how the weather has changed throughout the school year!