First you need your kids to have a will appointing you as the grand kids guardian for all things - their persons, finance, education, health.
If they don’t have that, they are basically wards of the state until you get it sorted out. And you need notarized possession of that too.
Work it out with your kids how things would be handled if they get sick-do they have a friend that could keep them till you come and get them? Etc.
Thank you for your response to Melian. Good advice for many of us grandparents.