I believe the day was exacerbated by a sticky seatbelt that little Summer couldn't release herself.
It always struck me, in CW's account of the day, her specificity -- helped Summer with her seatbelt and she popped right up.
And what's that rule-- let sleeping babies lie (just not in hot cars)? Warm day at the lake, warm car, too warm zip up outfit.... if she left Summer in the car because she was sound asleep... carried in the groceries... helped Grandma carry hers in, help adjust her knee brace, lose track of time...
Truth: CW unbuckled the seatbelt.
Summer didn't pop up.
JMO