Yea? Compare him to Boa Hancock as a child then. The eyes of Crocodile aren't round and girly enough.
I'm not saying it's impossible but there's really nothing saying he was a girl since even his kid picture looks like a boy.
If it didn't then there'd be something to discuss, but I feel right now there's just no substance to this popular theory
Compare him to Koala, when she was first introduced, even the scanlators weren't sure whether she was now supposed to be male or female, that only became clear after a few chapters.
Mihawk could also easily have some 'female touches' if you would change a few things when he was little. Forget that he was ever male, put him in female cloths when he was a kid and you could see a girl in him.
These are kids, so yeah it's an invalid argument to start saying that he wasn't girly enough when he was a kid as apparently you only have very girly girls and very boyish boys according to your logic, while in fact there are people in between, girls that look like boys and vice versa. What you said does not invalidate that Crocodile might have been a girl when he was little. What you say is already invalidated on itself because it are little children who lack the most fundamental traits of distinguishing genders: boobs and hips. And if you assume that Crocodile was a woman, then you need to ask yourself why he changed genders? If it was because he considered himself more man than woman, well it wouldn't be very weird for him to look like a boy when he was still a kid and there are quite a lot of such people in the OP world that feel that way. Let me recapitulate:
1) We have a character that has the power to change people's gender completely and he uses that ability like it's an Olympic sport. He's also very famous for doing so.
2) That character, Ivankov, met Crocodile and he met him on a moment Crocodile was still an upcoming rookie, meaning he wasn't that well known yet (and thus could easily swap genders) and we know that Ivankov came to know something about Crocodile enough to put him under pressure. Gender exchange could do that.
3) During Roger's execution his face was the only one not shown of all the future Shichibukai that were there. Moria, Mihawk and Doflamingo were all there and we could see them all from the front. Idem for other characters like Buggy, Shanks and even Dragon. The only person who was portrayed in such a weird way was Crocodile, we only saw him from the back and then only a small part of his shoulder and head. There is nothing in that picture that could say which gender he had.
4) As a child he looks like a boy, but I can also easily see in him a grumpy tomboy. That's really not hard if you just focus on his face. Then you also need to take into consideration that it's very well possible Oda did that on purpose. It's not unusual for him to, when drawing the child versions of many characters, he gives subtle hints and references to what they later became. For example Kidd who was playing with metal toys, Law who was dissecting frogs, Appoo who was playing the piano etc. so Oda might have on purpose made Crocodile appear more boyish than girlish.
It's not as odd as it might sound. I don't say this theory is true, then it wouldn't be a theory. In all honesty it's not that I myself seriously belief this was the case, however I do take into consideration it's far from being impossible as there is nothing that disproves it and it has a basis of being potentially true. Just saying that he looks like a boy when he was a kid isn't a valid argument as that can easily be explained.