In reply to john morrissey:
Think Cotswold do some Mammut kids down bags.
Also look bang on your requirements: https://kampaka.com/
Not down, but have a look in Mountain Warehouse - just bought 2 synthetic kids bags there for £20, half price. Pack quite small, but, obviously, not as small as a down bag.
The other option, as I'm sure you know, is to buy a womens bag, which will be shorter than a mens bag.