I had a similar thing happen to me with the twins and being at chiro. Put them into the car (2 door hyundai) and if you press the lock button accidentally without the doors being closed it will activate the mechanism and when the doors are all closed it locks. I didn't realise the locking mechanism had been activated when I was putting the boys into the car so finished with them and shut the door, went around to the drivers side to get in and door locked with kids inside. Luckily it wasn't a hot day, but I was freaking out... ran inside to get them to call RACV who took ages to answer who told me a twech was an hour away and they could call the fire brigade to break a window.
While we were waiting I went back to the car to check on the boys (as best I could) and searched around and luckily found the car keys had fallen onto the ground just under the car - disaster averted - called back RACV to stop Fire Brigade coming as we were all o.k, but now I have a lanyard with my keys attached which I put around my neck when I'm putting the kids in the car and I will always leave a car door open until everyone is all in safely including me.
I know on hot days putting the a/c on to cool the car down before everyone gets in seems like the best option but for me, I open the car up put the key in the ignition to open the window then take the key out, put it around my neck and then get the kids in the car and once we are all in safely then I'll turn the car on and crank up the a/c and then close the windows.
Glad you and your little one were all o.k.

