what kind of educational games?
I would go with tkinter (since you know it already) if its like
dark room or
cookie clicker. Its easy to make buttons and other GUI tools and there is not much animation and movement. However once a button class is established, these games could be easily replicated in Pygame.
I would go with pygame if it uses movement (ie. platformers, shooters, RPG's, etc.) Pygame Rects makes it easier to handle things moving all over the screen. It also has a decent user base when you have a question regarding gaming. This user base has also created custom map loaders for pygame to insert tilesets for games for more complicated game creation. However, pygame does not have a default GUI system. AKA you have to make a button from scratch. Usually pygamers have created their own GUI toolkit or use another persons if they continue to use pygame. But from someone just starting out trying to make a simple button can be daunting.
I would probably only use Kivy if it is on Android. Kivy is like learning a whole different language besides python as they use their kivy language. However this can put the game on mobile devices which a lot of kids utilize.
But it really depends on what you want to learn and find useful to learn as to which direction you go.