I think like most things ''it-could''.
I think the child would need to be in a certain headspace.
Some can sit and think, but not all are of that nature...most want to run around
play,and have a hard time even concentrating.
I have seen quite children, who could be taught...but to be quite honest, it's not the norm.
For those children, I believe it is quite possible.
As far as a subject, if it were me I would choose ''Logic''.
Of course it could be but as both a qualified primary teacher and a philosophy graduate, I can assure you it doesn't need to be - all children are born as natural philosophers!
The thing that schools and parents need to ask themselves is why they teach their children to STOP being philosophers.
But if you want good topics for discussion with children, try ethics - is it right to say sorry when you're not actually sorry, for example, or why sometimes telling the truth is not always the best option....
You can talk about storylines of favourite books and films and discuss whether someone is treated fairly or not.
Logic is also good, as children tend to be fiercely logical. I spent an hour in a university lecture once proving that fish do not have noses and yesterday was quite tickled to be asked by my son why some creatures have noses and some don't :)