I know two brands by head who sell sleeping bags for smaller people (1,60, 1,80 models in their range) suited for colder periods.

 -The North Face model Cats Meow comes in different sizes for smaller people.

- The Mammut/Angjulak models 3-Season Compact and the Winter Compact come in diferent sizes for smaller people.

I own two Mammuts 3-season compacts of 1,60m for my two kids (I don not trust them yet with the more expensive down bags...). These bags are good for a mild night frost and still weigh way below the 1,5 kg each. Check out the website for the exact specs. I found it better than the cats meow of TNF. The wintercompact was too havy to my liking and not necersaaryy for my kids. We only camp with them from early spring to late fall when only a mild night frost can be expected as minimal temp.

That's an option too but not really a weight saving solution. We did that in the beginning with our first kid and an old bag of mine. That bag was really heavy and old, so we figured it was better to save some weight.

 But instead of a belt you could use a piece of shockcord or a webbing strap.