In reply to John Kelly:
Yup, that's the one. It's worked great for us - maybe you had a slightly duff one. Alpkit were fairly cautious about rating it, I remember - they had a vague idea of +6 degrees, but were open about not wanting to push it.
We did a bunch of summer camping, then Autumn, some hut stuff in the Pyrenees and then ABC in Nepal (tea houses, but there was frost inside the room). Snow stuff, all fine. I'd say we've been to -2/3 with thermal baselayers and no complaints.
My own experience is that if you get the ground insulation right, the bag's absolutely fine: kids have a higher surface area to volume ratio, so they lose heat by conduction quite quickly. Whilst the fill power is only about 600, the loft is good and being a small bag, there's no additional dead space to heat (as is the case with a folded-over adult bag).
I think alpkit are missing a niche market here, with kids' and teens' down bags. There's almost nothing out there - off the peg - for children which combines warmth, low weight and packability. I had a chat with alpkit about it, but they weren't keen. Interested, but not keen.
This summer, we'll have the kids in the alpkid and a Trimm junior balance (absolute steal from sportpursuit a couple of years ago) respectively; we'll be at about 4000m in the southern hemisphere and I know from experience that we'll be fine.
But isn't that just the same with adult bags?