I'd suggest picking really easy veg - carrots are remarkably unreliable. Onions, quite frankly, are just really really boring to grow. Potatoes are great, they can chit them themselves, plant them, see the plant cycle, then dig them up. If you want, it's fine to do these in sacks. Try getting some unusual looking varieties - Salad Blue and Pink Fir Apple are two I'd recommend. Jerusalem artichokes aren't to everybody's taste, but gorgeous plants, starting as a funny nobbly tuber, growing 7+ft tall, then when they die back, fascinating to dig up. Put those in a corner straight in the ground. Definitely tomatoes, but do them in sacks or pots (growbags are great). Pick little varieties that your child can pick off and eat like sweets (and they are incredibly sweet compared to from the supermarket). Those will work best in your conservatory. Have a go at Swiss Chard, get some rainbow variety, the colours are amazing, and there are different ways to eat them. Plus it'll be something totally new as they're very seldom in the shops. These are best in the ground. Have you somewhere you can put in a couple of summer raspberry canes? They're great for a child to walk amongst and pick the fruit the adults can't reach. Have you a spot you can put in some garden beans, growing up a pyramid of canes. Start those off indoors in little peat pots, but then plant out when they're about 6" high. Beautiful flowers, and the beans just keep coming in season.