some “secret” seeds, potflower (a glass if you want see more clear), soil, label
To make more fun, give the children a seed that they do not know its name. It can be because they never seen the seed before (or maybe if they already know about the seed, try to find another from different place – local variety seed sometimes looks different). Later you can also try different culture method like cutting, leaf planting to know different part of plant that can be grown (intermediate experiment)
Give assignment to children and you can make a contest, which children or team will correctly guess the name of the plant that they grow. Before they found the name, they can create their own names (don't we remember that children love to name their own “pet”?) like : Tiny John, Rufus americana - this is if you want them to be familiar with binomial nomenclature things :).
Give presents for instance : handicraft from recycle product (cool isn’t it?) to first, second and other winner. We can also make other story to tell children about the habitat of the plant with photos, movies and etc if we want to make the happiness continue.
Goodluck for your experiment...