I think at that age, the key messages are the same as ever around sexist stereotypes, there are no such thing as boys' toys and girls' toys, they're just toys and you play with the trucks or the kitchen if you want (and challenging any ideas to the contrary they get from school).
On gender identity ideology, I would explain it in exactly the same way you might explains Christians or Muslims to a child if you're not one - some people believe x, and they're perfectly entitled to do so, so we respect them in their beliefs. But obviously that doesn't mean you shouldn't believe your own eyes or trust your own instincts, and if a male-looking or sounding person is trying to follow a girl into the ladies' loos at the playground, then come and tell me, don't go in there. That kind of thing.
I don't think I would bother with buying specific children's books for them, as it needs to be more of an ongoing situation throughout their childhood as part of just learning about the world, keeping safe, navigating social situations, etc. It's probably more important that you buy books for yourself on the topic (Stock, Joyce, etc) if you haven't already, so that you're clear on what's what and the arguments on both sides, as you don't want to be blindsided if a teacher suggests your kid is trans or when you need to speak out quickly to challenge something in the moment.
I find that once I feel clear and informed and have drawn my own conclusions on a topic my DC might encounter (e.g. online freedom vs safety, media literacy, etc (my DC are older)), and I know enough of the risks and benefits and the various arguments, I'm much more confident in navigating the unexpected situations when they arise, which is the important bit. Obviously it's fine to admit to your DC that you don't know something and need to look it up, but I think on gender identity ideology especially, the idea and purported freedom it promises children is so enticing that you need to be ready with what you think, there and then, as it's simply such an appealing concept to children and it can take hold really quickly.
Obviously, you would teach them about their bodies and growing up anyway, so it's more a case of checking that the growing up books you choose to explain sex differences (and fiction more generally) aren't full of woo.
The other thing I would say is that the more I know about how gender identity ideologues think about gender and sex and about the language they're using (which is constantly evolving), the easier it is for me to spot when my DC have been consuming content around those ideas, so it's a handy early warning sign. It's similar to how, as your DC get older, you'll start to pick up when they've been exposed to Andrew Tate-style content from the language they throw around in random conversations at home. So just understanding what they might be exposed to is really helpful in recognising it in its more subtle forms and addressing it as it arises.