You can put them in sooner but you must be willing to protect them from frost, by covering them on cool evenings. There will be tasks to be done all summer long. I give them each their own watering can to use, then I partly fill the kiddie pool. What you harvest, depends of course on what is ripe at the time. You can pick flowers all summer long for arrangements and pressing and drying.
Berries are great straight out of the garden or in desserts and jams.
Fresh veggies cannot be beat for taste and nutritional value. Let the children help in the preparing of the food to build on their interest. In the Fall bulbs can be planted. The garden will need to be cleaned up and the children can help bring the leftover plant material to the composter or in my case the pile in the bush, behind the fence. Small or child sized tools can make any job in the garden more fun for the children.