I remember the Blue Peter garden thing, it was very sad.
At school we had four raised beds about 1.5m x 3m each plus a load of random containers from the tip (old tyres, baby baths etc). We ran the club at lunchtimes because our own children weren't that keen and would have had to hang around after school if we'd done it then. Also if it had poured with rain we would have had to entertain them indoors, at lunchtimes we could just cancel. However doing it at lunchtime meant the attendance was quite variable (even though they were signed up as a formal club). as it was tucked away and they would just forget to turn up some weeks. We were able to claim some money back for reasonable expenses, but in reality quite a bit of stuff came from our own gardens or the tip (old pots, seeds, seedlings, old tools, old water butt, old hose, old mini greenhouse etc). Our raised beds and a shed were funded by the PTA and we got some kids tools and mini watering cans with the Sainsburys Active kids vouchers. We ran from Feb half term to Oct half term.
Growing wise, we went for things that would either crop quickly/early (radishes, broad beans, strawberry plants, lettuce etc) or could be left over the summer (beetroot, pumpkins, squashes, maincrop potatoes). So we avoided tomatoes, courgettes, runner beans, the things that crop heavily in August. We did let the children grow their own tomato plants on their classroom windowsills to take home (had to do a few spare for anyone whose didn't germinate). We had to come in and water over the summer and one year over a hot Easter too, we did manage to rig up a water butt and gutter to our shed but also had to use a hose quite a lot of the time, which was a PITA as the tap was about 100m away across the playground so could only be done when the kids weren't around. We did find the easiest way to keep the kids occupied in spare moments was watering with the little cans.
When we did get a particularly good harvest we would come in and help the children sell it to parents at PTA events or after school, but mostly they just took home little parcels (there was a limit to how much time we wanted to spend on it all). In the end (after about 3 or 4 years) we gave up though, it was a big commitment of time and effort (we were two main organisers and always had to rope in other helpers) and it all got a bit much in the end.
Anyway - what have I been up to? Watering mainly. even the weeds have more or less stopped growing, Everything has stopped growing- nothing is dying, it's just suspended. My courgettes which have been out for 5 or 6 weeks are no bigger than they were when planted out. It's very sociable up at the plot though as everyone is going up late in the evenings to water.