My best recommendation would be a runner bean, such as Insuk's Wang Kong. The vines will climb virtually as high as you let them, with red flowers (which attract hummingbirds), and large purple or black seeds, so your toddler will find them interesting in all stages. As a bonus, you can cook the young pods as green beans.

There are several white-flowered runner beans that are just as vigorous, and have huge white beans when mature. Gigandes and Bianco de Spagna are two good ones... and there is even one named Jack & the Beanstalk (sold by Peace Seedlings) which they claim will grow to 20'.

You could grow some Asian yardlong beans. They get very long pods, 2' or more when mature. There is a purple podded variety, Chinese Red Noodle, that is widely available.

Last but not least, you could grow a mix of green, yellow, and purple podded snap beans together. Besides the visual appeal, your toddler could watch the "magic" purple beans change color when cooked, and probably enjoy eating the snaps.