Are there any flower seeds besides marigolds that young children ( under 9 years old) can sow now in July and have them to bloom before fall?
Did you make a selection? If I'd seen this sooner I would have suggested some of the smaller zinnias which will flower in a little less than a couple of months. You also might just opt for buying some chrysanthemum plants that aren't in flower yet. The kids could plant them, and still have the pleasant anticipation of seeing what the flowers would look like.
For my kids, I have sensitive plant seeds for them that i got in a trade, and they're having fun watching them grow. They will be houseplants and won't bloom for this year, but kids love them! They're also called touch me nots, and he reason why they're called that is because the plant's fern like leaves close up when you touch them.
For outdoor plants in the fall, some garden centers sell black and orange pansies for halloween. They're good for fall and they will come back in the spring and die back in the summer. Just be sure to collect seeds to save money so you can grow them for the following year.
I see that you also live in MD as so do I. Check out valley view farms in Timonium. That's where I bought my black petunias, and they always have something unusual there.