Bagpuss yep def sounds like it 😕 dd was awful at that point, it did settle after a certain point but was terrible at the time.
Whatho you're doing so well, personally I would only use double cream to make the porridge with, you can also add seeds like chia seeds which don't add bulk but do add a few calories.
Do you make the smoothies? I would make them with fruit and double cream, a small amount of juice to get the right consistency if needed.
Would she have a small piece of cake instead of a cookie? If you can home made cakes and biscuits will be more calories, flapjack is great as easy to make just oats, butter and sugar.
High calorie dinners I made were ones with a white sauce so fish pie, creamy pasta, pasta bakes etc. The white sauce would be flour and butter made up with cream and ff milk, add in cheese and soft cheeses. The highest calorie soft cheese I found was mascarpone.
Curries are also good as coconut milk is very high in fat/calories.
If possible try adding a pudding with dinner, we did that instead of an evening snack. Pudding would be like the old school dinner puds, sponge pudding and custard etc. If that's a definite no go then maybe another smoothie/milkshake before bed?
An example of what my dd was having a that point would be:
porridge with double cream plus small glass of juice- 700 cals.
Piece of cake - 250 cals
Lunch would be scrambled eggs on toast (made with double cream) and a yogurt or bag of crisps (other lunches were tuna baguette, beans on toast with grated cheese, homemade soup made with double cream) 700 cals plus
Snack - cereal bar 200 cals
Dinner and pudding as as above 1200 cals
Milk drink - 150cals
So over 3000 in total.