in a real emergency, the sort of tunic dress made by folding the fabric in half so the fold becomes the shoulder seam, then lie the child on it and draw round her arms and upper torso, (leave plenty of room)- then flare out to the hemline, can be made in about an hour flat. Its very simple, two seam lines plus hemming neck, hem and wrist, and with a belt and some accessories won't look horribly out of place in most medieval contexts. It won't be perfect stylewise, but it won't be glaringly odd either
eta- this page explains it better.
http://www.vogthandcrafts.com/sca/ktunic1.html consider this a last minute emergency dodge though, not the ideal. Its a method that has been known to get lots of people through similar emergencies though so you won't be the only one