How to make sure I don't overfeed my almost 2 year old whilst feeding up my 3.5 year old?

[Flowerpupp]

My 3.5 year old has had issues with her weight since being weaned and is still being monitored by the Dr due to dropping from 25th percentile to 2nd. We have been referred to the dietician (who dismissed us quickly as said we were doing everything that she would have advised), had tests for coeliac, etc. She eats a wide variety of food but it can swing from eating loads to next to nothing, she's also incredibly active and doesn't sit still for long. People regularly comment on her size and weight.
My 1, almost 2 year old, on the other hand was born at the 90th percentile for weight and has remained there or thereabouts (but is only about 50th for height). She loves her food and whilst being active due to us taking her out and about and playing a lot, doesn't have the natural non-stop energy levels that my 3 year old has always had.
With my 3 year old I've never had to deny her food because we are always trying to encourage her to eat more. My younger daughter on the other hand will constantly ask for more until I feel like I need to tell her she can't have any more (like today when she ate 2 bananas one after they other and then wanted another so I said no as I felt like it was too much).

I don't want to give either of them a complex around food but the reality is that I have a 3 year old who needs to eat more and we need to get far more calories into her and a 22 month old who doesn't, but how do I encourage my older one to eat, try to give her higher calorie foods whilst saying no to the younger one and making sure that she doesn't overeat? I try to do things the don't see, like cover my 3 year olds veg in butter but not the wee one, and sometimes I'll be able to sneak my older one snacks when the wee one is busy but it's not always possible. Sometimes when I'm giving them both snacks my younger one will have hers and then pinch the 3 year old's snack if she hasn't eaten it yet. Their dad thinks that I should just focus on feeding our 3 year old up and give it to the younger one too and if it's too much for her and she becomes overweight then we can address that when she's older. I don't really agree with that but I really don't know how to maintain the balance. Does anyone else have any advice or ideas?

Can you serve the same things but make one more calorific?Eg add butter/cream/soft cheese in to things like mashed potato for 3yo? A spoon of soft cheese into tomato pasta. Full fat yoghurt, milk. A layer of full fat hummus/avocado/nut butter added to a sandwich?

This is pretty much what I try to do but find that okay for meal times but snacks are a bit trickier also because my 22 month old will have eaten much more at the previous meal so won't necessarily need a snack whereas my 3 year old will, but my 22 month old will want to eat any food she sees going, regardless of whether or not she's just eaten a big meal.

Ah I see, snacks must be tricky. Hopefully someone will be along with something helpful!
could you put a snack box in the fridge for each, each day and once it’s gone it’s gone? So 2yo knows the limit and 3yo knows the “target”.
have they suggested pediasure drinks or anything? 2yo wouldn’t need to know its a special drink if it was in a lidded cup.

These are good ideas.

To be honest oP, so long as they have the same things on their plate, I’m not sure the little one will be all that aware of say 6 baby potatoes vs 4. It’s super obvious to us but at two she won’t be that quick to count them in a pile.

I like the snack box idea so will try that, thanks.
We got paediasure before but she wasn't keen (I even got an ice cream maker and made ice cream out of it), I've seen that they have juice versions though but they're really expensive. Waiting the the next dr appt and was going to speak to them about it then and see if I can get them through the Dr.

Hopefully they can prescribe them for her. My adult dd prefers the juices to the shakes, something the texture!

Oh just a thought, if you have the pediasure powder, could you add a spoon to porridge or yoghurt? Or maybe add it to nut butter/choc spread to dip fruit in?

I would give your older child their snack where the younger one can't see it. Do you have a stair gate still? If so, older one can play upstairs with their toys for a bit with their snack (cheese cubes and crackers or something not too messy and not a huge choking hazard) and keep checking regularly. Or you could set the younger one up with an activity (painting or drawing) in their high chair facing your for a bit and then give the 3.5 yo their snack in the living-room where you can see them over the high chair but the little one doesn't necessarily notice.