If you're looking for a quick easy and fun sewing project your kids could get involved in, look no further. My daughters and I chose a couple of flannels from the Fat Quarter Shop (who have an impressive array). We ordered 2 yards of each (for a child up to about 1.4 metres; taller than that, you'd want to order more). Bought a pyjama sewing pattern and set to work.
One daughter was interested in sewing. One thought the idea was a complete waste of time. But the ten year old who was interested was able to pin and cut the pattern and sew the main seams (although needed help with the curved crotch seam). I hemmed and inserted elastic. Each pair took about an hour or two.