Most people propagate dahlias by dividing the rhizomes when they die back in the fall. This is much easier and more dependable than trying to cross-pollinate and start new dahlias from seed. They would most likely take 2 or 3 years to bloom from seed.
Where you had a single rhizome when you planted them, when you dig them after the foliage is finished, you will find a clump of rhizomes which you can separate and plant next year. They must be dug up and kept indoors to prevent them from freezing. Just storing in a paper bag in a cool, dark place will be good.
I too work with children, at a vegetable garden on the grounds of the local elementary school. They truly are fascinated by how things grow, and we 'stretch' their imaginations by growing different colors and shapes of vegetables - yellow, white and striped tomatoes, a rainbow of carrots, purple and orange cauliflower, rose, purple and yellow beans.
