This winter we're spending a month on the Bibbulmun Track with our two kids, who will then be 11 and just-turned-eight. They're great hikers who've had quite a bit of multi-day hiking experience here, in NZ and in Europe. They both carry all their own gear (but the youngest carries no food other than a bag of scroggin the pocket

We need a new sleeping bag for him, and he's pretty short for his age. Up until now he's used one of my husband's old down bags, but it's about 17 years old, and the down is very thin. He also sleeps very cold. When it's gotten too cold in the past he's rugged into my bag and we've been okay- we've also had him in lots of layers for bed. I'd rather avoid all those layers on this longer walk if possible; and yet he's now getting too big to comfortably fit in with me.
The temperatures can get down to around 2 degrees at night- last year we woke up to a dell full of white frost. So I'm curious about short bags, and have heard rumours about things such as half bags. I'm trying to save weight and space in his rucksack. In case it has any bearing on things we are usually in a ZPacks Triplex. My husband has a tarp he likes to be under outside, but (believe it or not) he fits in with us if there's rain about. With the four of us in there we manage to stay quite warm... but of course the rainy nights are also the warmer ones so that doesn't really solve our problem.
Any ideas or suggestions would be greatly appreciated!