What ultralight pack for my 11 year old daughter – one week village to village hiking in France (August)?

You have tons of options. You won't really have to carry much water, or food obviously. It sounds like her load will be just more or less her clothes and maybe rain protection and minor items.

Personally, I wouldn't bother spending $300 on an ultralight cuben fiber pack or something like that. You would have to buy it smaller than you could use and you have better options to shift weight (e.g., you just carry more).

I'd recommend some of the lighter options in the daypack category. One option might be those camel bak hydration packs, I know that my 10 year old thinks those are the most awesome invention ever. But I would probably just save the $100 and buy a $40 junior backpack like the REI tarn 18 or the Deuter Jr. pack, both are lightweight.

You might want to consider the Osprey Talon 22. It weighs 1 lb, 12 oz, more than you need but it will be very comfortable for your daughter in the small size and she can use it again in the future on simple overnights, etc. Great pack and one I used for my 10 year old son last year. It's also just $100 and full of pockets to help your daughter organize her things like jacket, camera, water, snacks, etc.

My son really likes his REI Stoke 29

http://www.rei.com/product/827115/rei-stoke-29-pack

1.5 pounds.

I would take a look at GoLite. They are affordable and super lite weight.

http://www.golite.com/packs-and-luggage/backcountry-ultralite

+1 on the GoLite packs.

My son's actually about the same size, 5'2" and not quite 90lb. He carried my old women's Jam2 on our last hike in the Grand Canyon with a little over 20lb.

maybe look at an REI flash 30 pack.
On sale at the REI outlet now for $50

Otherwise, I'd say a Jam35 is a pretty good option.