My littlest has shown a lot of interest in learning to sew; she makes things alongside me whenever I'm working. So far it's mostly been handsewing -- buttons onto scraps or small shapes into doll "blankets" -- but she's been asking to use my sewing machine for a long time. She's five right now (and a fairly mature five) so I decided it was time. Our first project is tissue cozies for teacher gifts and it's been perfect -- all straight seams and not very many of them. Plus they don't take much fabric so I get to make a dent in my overflowing scrap bin.
Both my girls have gotten involved (my eldest is 8) and they have been enjoying picking the fabric and, because each one is quick to sew up, the sewing, too. I don't think they'd have the attention span for anything more involved than this right now. We've sewing up one or two and then they've been decorating them with fabric paint and markers. They're really proud that they've made them themselves and have been compiling a huge list of people they want to give them to.