From someone that's been there, try to work on that mental switch. Even if the kid sees you as an exception, and really doesn't care if you don't, it's still a powerful thing to do it anyway. You can still crack jokes together and fuck around, but if you put in the effort to shift your language the rest of the time, it seriously uplifts someone that's in the process of finding their path as their authentic self.

And yeah, it is true that a decent number of kids in particular (as opposed to folks that are transitioning as adults) don't care about pronouns when their loved ones support them in every other way (or even just in most ways). They know some old fart has decades of brain patterns built up, and years of thinking of them as a given gender. They know it won't happen overnight, and thus aren't hurt by slips, or are hurt way less. But I've never met a trans kid that didn't feel happy and loved by a close family member putting in the effort anyway.

I promise you, the worst that can happen from trying is you get razzed for fucking up