It depends on where you are based. If you are in the UK, my 12 year old used a 25 litre day sack from Tesco, it's their own brand but has a padded, properly fitted hip belt and facility for adding a bladder if necessary. It also has lots of accessible storage pockets and mesh pockets, is well balanced and lightweight. Best of all only cost about £15. My 10 year old used a 18 litre one from Mountain Warehouse, again with a proper hip belt and bladder facility, I think it's supposed to be for cycling or fell running but fit her well and was again very lightweight. It was a little more expensive, around £22 a couple of years ago.
Also in the UK and mainland Europe, including Spain, there are branches of Decathlon all over. They have some excellent gear and do some great small rucksacks. Again they are their own brand but good quality and definitely suitable for the Camino and are extremely reasonably priced.