Don't forget to plan for your own death (i.e., contact funeral homes, cemeteries, etc.): Here is one of the many online resources to help (not an advertisement).
If applicable, don't forget to consider your other other children:
Consider the impact if they do become the legal guardian (i.e., are they up for the task?)
Consider if they do not want to become the legal guardian--be respectful of their decision.
Place legal, important documents, trust in ONE place and notify a trustworthy relative, friend, or lawyer of their location
Create advance directives for your medical team:
Example: Do you want to be an organ donor?
Example: Do you want life-sustaining treatments while in a coma or not?
It is perfectly alright to feel overwhelmed or other intense emotions; do not hesitate to consider therapy or other supports.
Enjoy your time as a family together.