The Bible tells us that God is always at work helping us to have the food we need. Jesus, as Lord of the Universe, created all things for himself, and "in him all things hold together" (Colossians 1:16-17) - so that means the plants that our food comes from too. He made them, they "hold together" - they grow, and keep growing - because of Him. Jesus makes it clear that we're to pray to "Our Father", "Give us today our daily bread!"
So it makes so much sense to praise Him and thank Him for our food - when it's Harvest, and all year round! At our church we're going to be having a special "Harvest" All-Age service, and I'll be doing Harvest Assemblies in school too. Perhaps you'll be celebrating Harvest with the Sunday School or a Kids' Club?
Here's a great song that sums that all up: "Harvest Time" - a time for "Thank you" because God is at work in it all, and "Please" because he's still at work today.