So, before I go back to CATch up, I want to mention I'm trying to get into her head, like really get into her head, and imagine scenarios.
So, how about this (still waking up, sorry if fuzzy post):
(SPECULATION/BRAINSTORMING)
They go the park. Lucas takes a hard hit or fall as a result of her VIOLENT HANDS. And dies. She scoops him up and puts him in the car. Either drives back home and keeps him in the car overnight. Then takes him (where)...
OR: BUT say he died at the park, she scoops him up, and throws him off the closest bridge or down the steepest canyon (you guys are more flat though) or pulls over down a side road about 10 to 20 minutes off a main road (my guess based on some other cases, of course could be longer, or shorter, anyway), places him in a ditch type area along the side of the road, or a field, a corn field (do you guys have corn fields) or abandoned farm or barn or in some trees/brush along the side of the road. Quick and common disposal (insert stat of bodies being close to the road).
Also, Mark Klass mentioned working one's ways out circular, stats of a. 1/2 miles and b. 5 miles
(Also note to insert Rossmo quote re: circular sprinkler)
*****
If he died at home (more likely, moo) then...?