Genuinely not sure here.
DD is 3.7 and I have never used the word fat around her or ever really. It's just not a word I like because it seems to only be used to put yourself or other people down, it's rarely used in a positive context. Back story here is that I had a mother who called me fat from a very young age so maybe this is clouding my judgment.
DD would snack all day if she could but we get loads of exercise so she's fine for the moment but I reckon I'd have to keep a quiet eye on her diet. She has no concept that eating too much makes you fat etc and I'm not sure I want her to but on the other hand there's my friend who takes an entirely different approach which I admire in a weird way.
Friends DD is 4 and my friend will often tell her "you'll get a fat tummy like me if you keep eating like that", "all that food will make you fat" etc, now I don't like the self deprecating or giving her DD and unrealistic idea of fat because my friend definitely isn't fat but I do admire the jovial but factual and open casualness of it all.
I know it reads like a total non issue, it's only a word but really it's the host of other things that go along with that.