Do you blame his wife for their son's kidnapping?

I agree not a smart thing to do, I was allowed to roam free from about age 11 onwards but 6 years old is nuts!

Speculation.
Why would the security guard throw minors out of the store without notifying their parent/guardian?

This was common back then. I was always left by the toy section growing up.

The Rules were simple:
1. Look but don't touch
2. Do not wander off
3. Do not talk to strangers

Aside from being forgotten in the store a couple times, it worked.

He was relatively safe in the department store. If anyone is to be blamed, its the security guard.

All moms did that up until this case got publicity. Things changed really quick then. In the early '70s at 5-6 years old I was left alone all the time in toy departments and there were usually lots of other kids there too. Ten years later in the early 80s when the Adam Walsh case happened, my nephew was about that age. Then, you would never see kids alone in stores.

I was walking to kindergarten and back alone at age five. At age six I walked half a mile downtown to the park, stores, library, and movie theater all the time. This was common in the past, for all kids. His mother did nothing unusual or wrong. Child abductions by strangers were, and still are, extraordinarily rare.

What happened to the other 3 kids? Did they ever talk about that?

Here's John Walsh describing all the details of the case with Leslie Foster https://youtu.be/JCuVX9keaE8?si=PGqF0qpiv86Mvjok

With Larry King https://youtu.be/Dm69xmIqMbE?si=WZaFnJ5Pb109N8Ri

With Geraldo Rivera https://youtu.be/AYCXhtTqS7I?si=6PpfsSmbMLHEFrG7