One of my favourite things to make for Amélie is a shirred dress. They are relatively quick and easy to make, and fit for a long time. A couple of her older ones have been worn as tops as they got too small to be dresses anymore.
You don't need a pattern, just a rectangle of fabric. There is very little wastage as I use the whole width of the fabric and just square it up. This particular dress was extra thrifty as I purchased the fabric (poly-cotton) which was already cheap, on sale and I used (Spotlight) vouchers as well.
Do you have any easy things that you like to make over and over for your kids? Particularly pattern-free things?