There's certainly a strong case to be made for children beginning needlework at the ripe old age of 5 or 6.
And with the emphasis these days on unisex activities for kids, your son can have just as much fun with a needle and thread as your daughter may have already had with a hammer and nails.
Once your get started as the teacher, it won't take long to see that you might take a few lessons from your pupil. One, anyway - spontaneity. Children are original when it comes to art. They haven't had years to "clutter" their minds with preconceived notions and guidelines about color, design and subject matter.
The plastic canvas that's available at all needlework stores is an absolute natural for the young. Because it's stiff, one small hand can hold it easily while the other hand just pokes with a needle and thread in and out of the large, clearly defined squares of the canvas. (Another plus: with plastic canvas, a large, blunt needle is used, so even the most accident-prone child will have a hard time getting into trouble.)
Once the poking begins, it will be only minutes before all sorts of "one-of-a-kind" stitches unfold onto the canvas.
Crewel is another terrific adventure for children. The natural way to start out would be to transfer one of their drawings onto linen, but I think I have a better idea. Stretch some linen onto a stretcher frame, hand your child a felt-tipped pen, and let her (or him) loose. This method's best because whenever adults "copy" children's art, they inevitably change something about it - causing its "once-in-a-life-time" naivety to be lost forever.
To introduce a child to some basic crewel stitches, buy some of those great crayons that let you iron a design permanently to cloth once the drawing's been done on the linen. This way, your small student can try some touches of embroidery in combination with the crayons, adding French Knots, buttonhole, etc.
One last world of advice: Whether you get your child involved in needlepoint or crewel, thread the needle double - never single - before you knot it. Otherwise, the teacher will be rethreading for the student every five minutes or so, and going quietly batty in the process!