Dandelions are a great plant and readily available throughout spring and summer after they first wake up in April.
Click on the photos to see them larger.

Speedy Germination

Day 1

Dandelion seeds can be collected by children, planted and watered.

Day 5ish
The first shoot appears.

Seed dispersal

Well this bit's obvious!

Demonstrating the purpose of flowers

Day 1
Pick two dandelion flowers and pollinate them by rubbing them together
You may buzz while doing this. 

Then leave the flowers in plenty of water.

Day 2
Watch as the flowers close up (very slowly).

Day 3
Fully closed.

Day 4,5,6,7,8,9,10 ish
Be patient. They will look dead.
Make sure they have water.

Day 11ish
Look the petal have fallen off!

Day 13ish
Opens up to reveal...seeds!
Oh! So that's what flowers do!

Day 15ish
Both are open now

What else can you do?

Make a dandelion timeline

Tell the time
Investigation - Can dandelions tell the time?
Easy to test!

Make a wish
(There is no scientific evidence that this works.)