Why don't you do it where you give, say, 2 people meat, fish or veg and then they have to find the rest of the ingredients off the lotties. You could ask other plot holders if they would mind their plots being used for this. The contestants would then have 30 mins to cook something up with random members of the public choosing the winner.
A small prize of a crate of stella artois or something?
You could also have it where you ask people to shout out a veg that has to be collected off one of the allotments. You could make like It's a Knockout with families taking part. Adults cooking and the kids getting the produce. You could then open a book on which kid will fall over first and make a bit on the side
Just a few suggestions. Don't know if they're much help. It would help to show that allotments are for families and not just blokes of a certain age.