If you're going to do object orientation, then stick with it all the way. I think a child would grasp OO far more easily than an adult with previous coding experience. OO is kind of the natural way to program, to me, even though I cut my programming teeth on Perl and PHP.
I don't know jack about Python, but it has always turned me off because of the followers. Anything a lot of people get very zealous about is something to be avoided, IMO. In the circles I'm usually involved in Python is talked about a lot, and if even half the stuff I've heard is true, Python is better than any language ever invented, past, present or future. There is NO WAY a single language can be as good for every problem as I've heard.
Don't underestimate children; they're smart. Don't pick a kiddy language like Logo or Basic. A child can learn proper OO techniques and Java or C# at the same time. Teach them what you're capable of teaching them, really. They're going to come to you for questions; be ready with the answers.