I am sorry for your loss.
Toothless was sick when you brought him home and it is very likely that if you get another bearded dragon from the pet store they will be sick as well.
I went back through your posts and looked at his pictures. His eyes look unwell. His posture in many of the pictures and the weird hiccoughing thing he was doing more and more while he was breathing suggest he was having problems breathing. His abdomen seemed swollen, but his limbs were thin and the bone at the base of his tail was protruding. His ribs were visible in at least one of the pictures where he’s doing the not-stargazing posture.
Part of his upper back and the base of his tail extending downwards towards his underbody and one of his legs look discolored in a strange way that made me think of yellow fungus.
If you can afford a necropsy, you should call a vet and have one performed. If he did have something like yellow fungus, all of the babies he was with at the store potentially have it. Before you consider putting another one of these little guys in his setup, it will need to be disinfected but knowing what you’re dealing with (infectious disease vs genetics) could help you make informed choices going forward.
I am sorry that you and your children woke up to this very sad experience this morning. Personally, I don’t think bringing him to a vet would have helped his situation and he did get to spend his final days with all he could eat, basking like a king in his spacious, interactive, and thoughtfully decorated enclosure.