I have a child that I truly need to fatten up 😬
With our fattening her up programme we can just about maintain the 1st centile. She's also got a serious health condition which may be the reason she's so tiny (she was also tube feed for a while as couldn't maintain her weight).
It doesn't sound too me like you need to fatten up yours, but that they are both quite petite.
But in terms of how we try to make her gain weight, we do:
Fortified drinks (on prescription) that provide an extra 300 calories a day. They have about 4 types of sugar in them 😳😂
All milk is gold top, so even more calories than full fat.
We try to add cheese to everything we can.
More biscuits than is sensible, and a lot of cake.
Hummus!
Obviously use butter not spreads, thick bread (or brioche), porridge made with gold top etc.
Hopefully we'll reintroduce some semblance of balance eventually, but for now, her dietician is keen that we get as many calories into her as possible.
Ps: she is in proportion, but there comes a point where even if it's proportional, some growth does need to happen.