IT is an IT. Not that I think the other comments are wrong, they just don't quite go deep enough into the lore and are a little too literal from the words on the pages of the book. For starters Mrs Kersh was IT, taunting Beverly, not a child of IT referring to their parent.
The second part about the giant female spider, seems to be one of the most debated points with this story, but here is my take on it. Its not exactly wrong to assume that section to be literal in some parts, but its missing some things, and you can trace it back to the smokehole chapter (bare with me, I'll try to be as succinct as possible): Mike and Richie experience IT coming to earth in pre historic times, seeing a physical creature crash from the sky to the earth. This tells us that IT took a physical form to come to our universe, and as we find out in later chapters, for IT to interact with our universe, IT HAS to take a physical form, it cannot remain formless and attack from the macroverse. Having said that, we know that IT absolutely exists in its true form in the macroverse, so we then have to take the premise as being that IT exists in both places at the same time, and are one in the same, but are abiding by the rules of their respective places within the cosmos - it implies that IT is more than capable of doing this, but they are intrinsically linked in spite of being in two places at the same time, you could view IT's ability to use the deadlights as a weapon, being an ability to open a link from its true self in the macrovrse, to the creature in Derry.
What this means for IT being male or female? it means that the giant spider creature in Derry, is certainly IT, but only what IT would be in our universe, having to abide by its rules while manifesting a form here. IT's true self in the macrovrse has no gender, but when and if IT decides to manifest into another universe other than it's own, it will become whatever that universes rules dictate it would be - the other forms over and above that are simply manipulations of the rules that IT is capable of, but are never its true version of what IT is when not choosing a form, which seem to enable IT the ability to wield a range of telepathic and magical abilities.
To summarize that all, IT is both the giant female spider, and a formless lovecraftian horror, but mostly the latter. The giant spider is just what it means for IT to exist in our universe, which you could view as a filter that takes the true self, applies the rules and then you get the form as a result of these things colliding - IT cannot choose to be anything else other than the spider when taking a physical form. The alternative is to be energy, formless, but this means that IT cannot interact with the physical space, which means that IT cannot hunt, so IT must manifest - first as the spider, but then can manipulate the rules, to then do everything else that it does. If that makes sense - it's not a quick concept to explain, and really ties into the whole lore around IT that King built into that story.