Hey, A....those chiggers suck!! You cannot even see them they are so small. They eat my family alive, as well. Unfortunately, they are prevalent in the Midwest.
I just found this wonderful stuff called "After Bite" that you can pick up at any drug store. You apply it like a magic marker pen. It stops the itch immediately. They also make one for children!
And, yes, bug repellent loses its effectiveness as time passes.
As for repellents that work, the old tried and true Skin So Soft by Avon works beautifully. Smells good, keeps your skin full of moisturizer and keeps the bugs off. Just need to reapply from time to time.
Best of luck to you!