Long story short, I’ve always been supportive of the trans community and individuals, trans rights are human rights, all that good stuff. However, I’ve never had someone in my personal life transition, until now; my FIL told my wife that they’re now a MIL. So there’s two sides to this request:
First, any general advice, resources, etc., on how to be supportive/helpful for her during the transition, and advice that would be more specific to someone that’s transitioning as a senior and as a trans woman that wouldn’t respond well to overtly left wing resources on the matter. Yep, she’s a lifelong Republican, was optimistic about Trump although I have no clue how she stands on him now (as an aside though, oh boy were the rants about drag queen story times ironic in hindsight).
Which leads into the second side: while she has started on HRT, she’s still not publicly presenting as feminine, and we haven’t told our son/her grandson yet. We’ve discussed the general concept of people who are transgender with him, but that’s not the same thing as him processing grandpa becoming grandma, and he’s at an age where I have no clue how he’s going to take it. So I’m looking for any recommendations as to how to discuss what’s happening with him, maybe some books (like third/fourth grade level) that do a good job of presenting the concept for a younger mind.
i’m sure you’re doing this but please take special care to make sure your wife has your emotional support as she processes; even trans-friendly people can find it dizzying to have someone who’s been central in their lives for all of their lives transition. my partner and mother have both been supremely supportive but they’ve both also had some complicated emotions about everything that needed to be worked through.
as for your
new MIL 
i’d say the most important thing you can do (granted, this is assuming your relationship is at least solid) is offer her a sympathetic ear - critically, one that’s fully private, even from your wife if need be. don’t offer if you don’t feel you can comfortably perform this role, but having a secure sounding board is imo instrumental for trans people starting out. as a senior of right-wing bent she may find herself hard up for people to turn to when she needs to talk about trans shit.
for your boy, i think just being frank and straightforward will be best. emma’s right ofc, you should consult with your MIL as to how she’d like to proceed, but personally i think he’d get more out of a discussion with a parent than he would something impersonal like a book, no matter how well written. i also don’t know any so there’s that.