Container gardenning is an option. I have a child with me who has a dark/dirty yard in the inner city,and so he plants a lot in pots here(at my place) to take home.
If there is an option for a garden plot at the school that would be great.
Worm composting can also be fun for kids.
They can get a batch of red wigglers at the pet store for $5,and have the worms in a plastic bin eating veggie scraps. Then they use the worm poo to grow say some herbs,lettuce,or greens. Don't need a lot of space for the greens.Plus they can learn the value of recycling containers for various uses.Greens don't take long at all.
And probably unlikely but a little pond is also great.It attracts so much wildlife.
Speaking of greens there is also the option of SPROUTING. I sprout alfala,radish,and broccoli.I think most kids would like the alfalfa.That just takes a few days,and kids find the growth of the seeds interesting,Speaking of sprouts they can also grow wheat and oat grass for the pets,but be warned they puke it.
Lotus seeds in a gallon puckle jar.Cool to grow.Builid a worm or root grow viewer with plexi glass and wood.Store ones cost $10-20 if parents want to buy those.
Ask the city for a city owned plot.The kids can clean it up and make garden beds for growing stuff.Hopefully no one would mess with it.
Kids love to build forts and stuff.You can even try to gow hide aways using sunflowers,morning glories,corn,various tall grasses.Maybe the school(or again the city) will provide a location for this.The school would be the best option since the kids are already there,and maybe they can just stay on a bit longer to do various outdoor projects.
The kids might have eldery neighbors with yards they have trouble tending too.Some eldery give up their gradens due to the difficulty,but would welcome help to get things going.It still isn't to late to grow the fast stuff.
Hoping you and the kids have a great summer! I would love to read an update to see what you(or the kids) end up doing.