"Failure to thrive" was a term used in its own right.
Children who were not "thriving" were also more vulnerable to mortality from infectious illness that hadn't got vaccine programmes, didn't have access to anti-biotics or were spread more easily due to poor living conditions.
A lot of mal-nourished children would have been recorded as other causes of death.
Anti-biotics weren't easily avaliable to the general population until after WW2 and co-incided with the creation of the NHS. Industrial slums were being cleared through the 1950s-1960s. The last time the question about indoor/ outdoor toilets was asked on the UK census was in 2001 which was when it finally became statistically negligable.
Taking indoor sanitation for granted as a default has been since the later part of the 20th century.
Central heating (and its benefits for health) is also comparatively recent although affording to use it is a seperate issue. It took decades for existing housing stock to be retro-fitted. I remember being bathed in front of a coal fire in the early 1980s. It must be one of my earliest memories as we did have gas heating installed after moving to that house.
Cold, damp homes made/make vulnerable people including children more susceptible to illness, particularly respiritory illness by constantly stressing the immune system. Even worse when breathing in mould spores.
Child mortality is now fortunately rare and rarely due to illness, and usually connected to conditions such as genetic/ mitichondrial differences, cancers, premature birth and complex conditions.
It's easy to forget how recent security of food, healthcare, warmth and sanitary housing are. Society isn't perfect, and even when families struggle at least children do get respite from problems like cold by going to heated schools for part of the day.
Food diversity didn't really gain momentum until the 1980s. Traditional British diets were pretty bland and repetitive so if a child found restricted safe food within that usual repetoire, it wouldn't have been so obvious as it is now. There would have been less access to variation of the same food type too.
I have a "sensory autistic" child. He's not AFRID league, but he is very sensitive to variations of taste and texture that most people don't notice.
I had awful nausea and food aversions in his pregnancy. I was rarely sick (oh how I longed for the relief of vomit) but I had a very sensitive gag reaction. I lost weight rapidly while gaining bump in the early months. There were several phases where I could only tolerate a couple of foods without triggering painful dry retching. For several weeks I existed on a certain brand of ginger stem cookies (fortunately calorie dense!) I joked that I could eat Pot Noodles because they weren't food. The meat aversion lasted longest- about 6 months. Everything in my head about eating well to nourish myself and baby went out of the window very rapidly!
When people say about AFRID that if it's a choice between unsafe food or starve, I very easily believe that they will starve.