Brown_Pride
All Star
And you know what, you're not going to know everything. But knowing what to do in teaching critical thinking skills isn't too difficult, read through that link, it'll take you through the know how on your end and even build your own skills.
However, "know how" really isn't the hard part, it's being able to implement the system until they get it. Which could take years considering plenty of factors (like your time schedule), but it will get easier as the time passes, because the longer they practice it, the easier it gets. Make sure you're not just telling them what to do, but make them actually do it (buy riddle books, etc.).
There's a Physicist by the name of Richard Feynman, his dad wasn't exceptionally smart (didn't get past algebra) but was able to teach his kid how to think. They too weren't wealthy, and couldn't afford the best elementary education. Plenty of the books Feynman wrote, he talked about how his dad would raise him, and how much of an impact it had on him. Feynman's IQ was only registered to be 125, and it's been generally accepted he may jot have had more raw brain power than plenty of other wcientiats, but it was his way of thinking that made many consider him to be one of the best and brilliant physicists this world has ever seen.
For sure i'm reading this, i actually had it book marked already from some other time when you posted it, i think i just forgot about it. I do already try some riddles with my kids, or in general just get them to try to think about things but the more resources i have the better. If you have any leads on any riddle type games hook it up.