Teaching both would work for older kids, but a lot of Papert's work suggests that most kids under 12 haven't yet developed enough capability for abstract thought to use explicitly mathematical concepts to model the world, so teaching FP style at that point is just going to lead to frustration.

If you're going to teach kids at younger ages, you need to introduce it in a way that they can relate to physically, which is why Logo makes the turtle central; it has a heading and position just like a child's body, and it moves in a similar way to the way you think about moving yourself.

Anyway, you should read Mindstorms if you're interested in this topic; it does a great job explaining why the Logo approach is specifically grounded in a thorough understanding of childhood cognitive development.