Posted: 24/05/2013 at 22:08
How big is the garden?
How big are the children?
How many of them do you have, and for how long?
Having just been in the "how to take honeysuckle cuttings" thread, I could suggest a scientific experiment to see what length of cutting to take and what fraction of it to bury for best results, but this leaves some children with the unsuccessful numbers feeling left out unless you've got enough room (and a big enough source plant) for them to take multiple cuttings each.
I can't recommend trying to grow fruit trees from seed. The current pupils' children may just get to see fruit. That's a rather long wait for rewards.
Plants grown from bulbs are pretty fast-moving things. Crocuses and anemones have been and gone, hyacinths have come and are fading and the gladioli are doing fine right now. What'll work for you depends on climate and drainage. Hyacinths don't like it soggy. Lilies seem to be a bit late coming, listed as "June-July" or "July-August" on crocus.co.uk, so they may flower too late, which would be a shame. Hyacinths will make an enclosed garden smell fantastic ... and just like concentrated bathroom disinfectant.
My previous computer is now serving as a karaoke machine, but with a £15 webcam and two pieces of free software (autoclicker and Gimp 2.2) it was able to record germinating seeds: http://s300.photobucket.com/user/Sableagle/library/Tree%20seedlings?sort=3&page=1
I wanted to do the same with a lily or hyacinth but never did. I still have the kit, so maybe I still can. That might be a fun project, but you have to make sure the camera's in the exact same place for each frame of a plant, meaning you basically only get to film one flower per year.
As an alternative take on it, how about a wildlife space? Butterflies and bees have been mentioned. Add hoverflies, ladybirds and lacewings to that, and maybe even birds. There are a lot of things you can do to attract them, and you could tie in environmental lessons.