i am so fucking confused, is cecil a girl or a boy ? and does josh have two sons and a dau...

I think the translator messed up on the two sons and one daughter if not the people probably didnt know or josh never mentioned it so keith/yeonwoo just played along, But cecil is not a girl they just dress him up as one. (sorry if none of this makes sense but cecil isnt a girl biologically at least.)

Yeah, In kiss me if you can, Chase, which is Cecil and Pete's Father, was seen dolled up as a girl when he was a child, so that may explain more onto why Cecil is seen as a girl ٩(๑❛ᴗ❛๑)۶

Maybe Cecil is a child superstar like his Father

i remember they have 3 children, one pete (m), second is cecil (m), and the third is a girl who is still a baby and never make an appearance

ohh okay i get it thxxx

In the last chapter of the main story (the one after the time skip), Josh has two sons, Yeonwoo was confused when Josh send him pictures, because Cecil looks like a girl, thought it was a daughter. But Keith told Yeonwoo, that Grayson told him that his brother (Chase, who is married to Josh) only had two sons. In the novel Josh mentioned his pregnancy with his third child in the phone call.

In the extras Keith is confused when he sees Cecil dressed up like a girl, so asks Yeonwoo, at that time Josh has two sons and one daughter who is too small to bring along.

Why Cecil is dressed up like a girl is explained in an extra where Yeonwoo and Spence visit Josh, that isn't adapted to manhwa, yet. Yeonwoo's second pregnancy and the babyshower are also missing.

Iirc Josh likes to dress him in cute clothes (Cecil hates pink by the way) and Cecil struggles with it, along comes Spence who thinks he is a girl who looks like a princess (and Pete like a prince) with a motivation speech. And you saw the result.
And Cecil is still a small boy, he is younger than Spence, it's just the Miller genes produces 2m+ giants, if he looks taller than Spence that's why.