You know this is a wonderful project. I remember when my son was about four, I wrote a hangman game where he had to guess rather simple words. He just loved that game. By the time he got to school, he was way ahead in spelling.
Here is one idea, put the text on a button, you click it and it changes ...
[php]# display a random sentence from a list in colorful Tkinter
from Tkinter import *
import random
black = '#000000'
blue = '#0000FF'
red = '#FF0000'
yellow = '#FFFF00'
lime = '#00FF00'
sentenceList = ['the dog is on skates', 'the bird is on a plane', 'the pig rides the horse']
def setText():
str1 = random.choice(sentenceList)
push1.config(text=str1)
#create the form
form1 = Tk()
# set the form's title
form1.title('Random Text')
# create a button
push1 = Button(form1, text='Click to set new text .............', command=setText)
# configure the button's text font and colors
push1.config(height=3, font=('times', 20, 'bold'), fg=black, bg=yellow)
# pack the button into the frame
push1.pack(expand=YES, fill=BOTH)
# start the event loop (run the program)
form1.mainloop()
[/php]
Pick the font, size and colors you want.