It's not hard. Lock the car and return the cart. Your kid(s) will be fine unattended in the car for 20 seconds crying or not. If this really distresses you, park in the back by a cart corral.
Menards does have huge cart corrals. The corrals have never been empty enough for me to confuse it for a parking spot, but without carts it would fit a car just fine. It's odd that they don't have dividers. Then again Costco doesn't either.
I had to check the Menards website. I knew Menards sold everything but it never dawned on me that they literally sell those carports. I didn't know that's what those structures were called either. That's a good point about the large lumber carts.