You don't get stamps but you can usually get one if you ask. I asked an aget at ORD who wasn't processing anyone at a regular lane if I could get a stamp. He looked at me like I was nuts for actually wanting a stamp (since most people don't want to waste space in their PP) but stamped it when I said yes since it was my first trip outside the US.

Your 6 month old will need a his/her own passport and be approved for GE as well. Everyone must apply for and be approved for GE before using the kiosks. It's not like the TSA's pre-check where kids 12 and under can accompany their parents through the pre line.