first thing you do is make yourself mobile. at their age, their values will come from you and how you live your life and your values will dictate so much of who they become relative to their relationship with the US as a country. how do you make yourself mobile? well, for me it's first mexican residency, which in turn gets my kids automatic residency. You'd likely want to visit often, but there are no actual residency requirements outside of the financial means requirements which are low for temporary and a bit higher for permanent. Costa Rica is another popular one but I find Costa Rica to be too remote, too under developed, great for a vacation, hard to make a permanent life there though.
from there it's a second passport. Mexico is one but it's like the US, tests, language proficiency, etc. there are several pathways to the EU, from Malta just paying cash for citizenship to the golden visas of Portugal and others. Malta is instant (once approved) citizenship for cash, the others are a 10 year process started with investment and residency first. Rules change periodically but I suspect US expats will always have a route in some form or fashion. the Caribbean island nations also have passports for sale (instant once approved). Be aware the instant passports are not cheap, they are usually well over $100,000 like $150,000 or something. But if you absolutely need one, it's a small price to pay to have global freedom.
with Mexican residency, a US passport and a second passport in the EU or in the Caribbean then you can travel pretty much anywhere and if the passport is the EU (or if you have EU nation-state residency from a golden visa program) then you can reside anywhere in the EU.
That should be a 3-5 year plan but by the 5th year you should have options that allow you to go when you want - i.e. have at least one residency visa outside the US.
from there, it's travel, even if modest it doesn't matter, open their eyes to the world. from there, it's looking at colleges in other countries like the EU. I can't get my kids to think about anything but UT but they are also near adulthood so we can reason with them regarding when to leave, especially if i can convince my ex to allow mexican residency "just because we can" right now.
foreign languages, focusing on emerging and dominant fields are all great, but it starts with you building YOUR framework for mobility and then your values for that mobility will nearly automatically pass on to your kids.
as for the criticisms for posting - fuck that noise - this shit is horrible. the amount of grief, fear, angst, anger, frustration, sadness, depression, fatigue, lethargy that the decline and the HATE causes is sometimes too much to bear. I'm sitting on our own applications for Mexican residency and haven't sent them to our facilitator for all of those emotional reasons and I have a clear and present danger in front of me that should have me sprinting to the goal. so post away and fuck the rest.
but again, focus on YOUR mobility and your kids will have it ingrained in them by the time it matters (plus it's a lot cheaper to drag them along as minors than it is for them to have to qualify on their own as adults later).