Can't quite make my mind up on this one!
It could be great for teaching mathematics, game theory, concentration, patience, decision-making etc.
On the other hand, while there's obvious free options, poker mostly exists for
gambling purposes. With my sensible head on, I don't know if would be a great idea to lead a kid towards gambling. Not as a kid, I wouldn't want them gambling then, but for the future. Do we want to encourage our children to go on to be gamblers as adults?
Having said that I gamble sensibly, most of us do. It's not a problem for us. I actually did gamble as a child too. From around the age of 12 I'd bet on horses with my grandmother, watching them on tv and sending my dad out to the betting shop to place our bets. It's not done me any harm in terms having having problems with gambling.
I know some people came to poker from other card games like Magic: The Gathering. I don't know that game but assume there's at least some transferable skills and that you could learn some of the same types of lesson. Maybe that would be better for kids?