wow, i was just about to postthe same thing, hadn't thought of pine cones though, brilliant idea.
i make salt douhg decs at school with my class and we just use PVA glue to coat them with, although varnish would probably make them last longer. You can buy craft "glaze" that is safe for children too.
My mum still has some pine cones i glittered when i was 5. I am now 30 and she keeps them in a tupperware box so they dont get bashed.