When a kid is in preschool, you want to find a kit that doesn't contain small pieces. At this age, I would recommend watching over your kids, just to be on the safe side. You never know when some glue or paper mache is going to seem like an appealing snack for a kid. Check out these ideas.
Primary School
Kids in primary school are going to be looking for activities that are fun for them. They're also at that age where they will want to be a little bit more independent. This means you don't have to watch over their every move, but be close by in case any accidents occur.
Pre Teens
Some pre teens will think they are too cool for arts and crafts, but wait until they see what you've brought home for them. They might be into makeup, mobile phones and ipods, but I think it's important for children to hold onto their childhood as long as they can. Arts and craft is a great way of achieving this. Check out these kits.