I'm not a Texas lawyer (California instead), but its law on trespass as reported by Texans on the Freep sounds similar to California law. The police can't make custodial arrests of people for trespass on private property in the absence of some form of authorization from the property owner. Posting a No Trespassing sign is generally sufficient, but often the property owners ask the police to act.
Nowhere has it been reported, that I've seen, that the property owners/store personnel asked the police to arrest trespassers or that no trespassing signs were posted (there wouldn't be those in parking lots - stores want customers to come in). Furthermore one of the news stories mentioned, and I saw in a TV clip, that the Houston PD chief wanted to know why the hell people were being arrested instead of cited.
All this together tells me that arresting ANYONE for trespass in these incidents is a per se violation of at least official police procedure, and most likely flat out illegal.
In the case of "trespass" after dark though, according to the laws you know, would it be ok for the cops to detain the kids, then call the property owner(or manager on duty) and ask if he/she would like to have the children arrested fro trespassing? Is it not customary for police, when observing suspicious behavior after dark, to determine what's going on? Wouldn't this entail contacting the property owner to ask if permission had been given to occupy the prperty, and if not, should the persons be arrested?
One more question? If K-Mart wasn't open 24 hours, and there was no restaurant next door, would this be different in your opinion?