I got involved in setting up a gardening club for a local primary school ten years ago and really enjoyed it. When I handed it over to a teacher after three months I missed it terribly (had to go to work sadly!)
Pumpkins, strawberries, potatoes, herbs, sweetcorn (swift started early on a warm windowsill), salads, courgettes, carrots and peas are all good things to grow (either because they can be harvested during term time, eaten in situ or are fascinating to grow/harvest.
I would recommend you do a sunflower growing competition, the heads can then feed the birds in winter time and you can then include a "garden birds" lesson in the process when there is little happening otherwise in the allotment. The children can build a windowsill light reflector in their art lesson to grow seeds on the windowsill also now ready to start seeds off. It is also worth seeing if you can get funding for things like pocket compasses or magnifying glasses so you can do a lesson in early spring before planting starts where the children work out what way the plot faces and where the sun comes up and goes down and where best to plant things, etc. You can also talk about the bugs and worms in the soil and what they do, etc. Same for what type of soil - get them to analyse the soil type by pick up soil and squeezing it in the hand to see if it sticks together (for clay) or not. You can also create things like measurers made of old cut up plastic cartons that children can put down a planting hole to check they are planting seed at the right level. The measure can just be marked with the word of the item you are planting and the top of the soil level.
I know there is a grant sponsored by Southern Water for allotments and community gardening groups which is for up to £5k so, if you are in a SW area it might be worth a try. Usually it is managed by the local Community Foundation - worth checking with the local council to find out who yours is.
I would also contact the local garden centres to see if they are willing to donate.
We also have the Friary Gardeners here who are a local gardening trust which help teach people with learning disabilities life skills as well as gardening skills, they are always happy to grow plants for community and school projects if asked. If you have something similar, you could ask them to start plants off for you if you have limited resources initially.
I am always amazed what efforts local people will go to help a community project so it is worth getting the local paper on board and writing a little "help needed" article.
When you set this up, ask the school what their rules are about photos featuring pupils. See if they will organise for the parents of participating pupils to sign an agreement to use photos for press articles or grant applications so you can take photos to send to potential future funders. It really helps being able to feature real photos of your project when making applications.
Oh and there will always be children who aren't that keen on the nitty gritty, so just get some cheap colourful watering cans that they can fill from the water butt as I used to keep children occupied for hours watering anything green within the vicinity when they were bored with planting!
As you can tell, I am pretty keen on children learning about gardening because it encompasses so many subjects and if you can grow food, you will never go hungry!