Erm, I'm just thinking about my neph, which is very good at playing "Memory". So here just comes as an idea for an approval of concept:

• create an array of 16 up to 25 images (more or less, just as you like) and melt them together as an imagemap that gets determined by ther server
• let the kid choose an order in which it picks a number of images (least 6, most 12), eg. the key now is "car, dog, shoe, key, kid, bicycle"
• the child may have a plain text as a login, but a password that is specified by cordinates, which can only be written to clear text if you know (see) the imagemap, which can differ every time, e.g. once you have a red greyhound (dog) and the other time a green shephard(dog)
• when the user (here its a kid) comes back, shuffle your images to a new map
• when entering the key (clicking on the map in the right order) the user can authorize

But ok, it's just one more idea cause I've got to much time currently.
Have a nice day
All decision is left to your taste