I've felt your pain. Dd is extremely slim I later learned not simply my family's weird genetics but her disability means she's taller than most and has a higher metabolism.
People think losing weight is hard (I'm overweight myself and have lost weight I'm not saying it's easy especially) but gaining/maintaining a healthy weight if you tend to the lean is MUCH harder.
In addition I have a child who hates chocolate, chips and much deep fried food! (Again found out later that some sufferers of her disability these foods caused her discomfort/irritation)
I've found full fat milk, full fat yogurt (murder to find these days), cheese, nuts, creamy dips (inc hummus), buttered toast, crumpets, eggs, root veg other than potato (she's not a fan of potato generally, as I say something I recently learned is it can be an irritant to some) all helped.
Plus encouraging to eat often as she can't manage big portions, so instead needs to eat more often.
It's really hard. She's 18 now currently touching on 6ft and a size 8 bottom half, wears a size 10 top BUT that's partly to do with being broad shouldered and wearing things on the loose side so she's not struggling to put them on/take them off - again to do with her disability. I think of it were purely on size it'd be size 8 tops too.
You're right on the sugar, not just because of the health but the naturally slimmer among us tend to metabolise it super fast anyway, if dd tends to the sweet rather than savoury she ends up losing weight.