The issue isn't that it couldn't happen to someone Summer's age, it's the time required for death to occur.
From the last time she was seen by someone other than the family, H, to when the 911 call was placed was about 3-4 hours. Also, the video of her sleeping was taken after the time H claimed he was dropped off. So let's assume they got home and left her in the car around 3:30pm.
The way kids die in hot vehicles is the interior temperature increases to dangerous levels which in turn increases body temperature. The lower the outside temperature the longer it takes. The high that day was 80F. Here is a calculator to estimate how long it takes for the heat to reach dangerous levels. Assuming the worst, 0% cloud cover, windows up, and dark car, it's estimated to take 65 min for body temp to reach 104F and the life-threatening level, and 105 min to reach the medical emergency level. This is based off a 2 year old child. After 25 min she would be sweating profusely and very uncomfortable. Again this is based off of worse case scenario. The property is surrounded by trees so the truck could have been parked in the shade, which would increase the time it took for the temperature to become dangerous. Also if the windows were open or cracked.
Back to assuming the worst. She would most likely need at least an hour to hour and half unattended in the car for her to expire, possibly longer. Even it's just an hour, that gives Candus 2 hours to dispose of a body outside the search area, that has still not been found almost 2 months later, get back and call authorities. That's assuming she doesn't freak out and panic first. She doesn't come across as the type to be able to think on her feet in a situation like that.
I typed "child swimsuit with skirt" into the search engine and lots of products popped up. Including skirts longer and more frilly than the one pictured above. Here is one option on Amazon.
I don't know for sure what Summer is wearing, but the top honestly looks like a swim top to me.
this one appears to be similar and about the same length as the one she was wearing. not the exact one, but just an example of the type of swimsuit.
(BAOHULU Toddler Girls Swimsuit One Piece Cute Floral Dress Swimwear 3-8 Years https://www.amazon.ca/dp/B085XZ2V8V/ref=cm_sw_r_cp_api_glt_i_HT0BZBETJ9RRJ83Z9PVS)