My son and I will be flying into Amsterdam and then changing planes to continue on to Brussels. We are traveling light (so we will have our carry on bags but no checked bags). Where will we be required to go through customs (Amsterdam or Brussels?)
You will go through at your first point of entry, which will be Amsterdam.
The immigration (passport) check will be in Amsterdam, the customs check (the stuff you're bring) will be in Brussels. You also have the option of taking a train from Schipol Airport to Brussels. There are trains every hour and will take you directly to the city center in 2.5 hours. Fare is 36 euros.
As Micheal mentioned, immigration and customs are two separate stations. But customs in the Schengen zone is just a walk through, unless you have something to declare.