One solution is to use a Donor Advised Fund (DAF), such as Vanguard Charitable or Fidelity Charitable.
You can name the DAF as a beneficiary of your estate. Within the DAF, you can set up a legacy plan.
I can't speak for all DAFs, but Vanguard Charitable allows you a few options:
1) Name somebody to manage your DAF distributions
2) Set up an automatic distribution to named charities
as a percentage of the holdings over years, like an endowment
3) Cash it all out at once per your charity list and percentages
You can even mix and match #1 and #2, so perhaps let your children manage a portion of your endowment as they like, but also retain your own plan.
The nice thing about this is it doesn't require you to update your will or trust. Just name the DAF and you can change the legacy plan anytime.
This does require confidence in the DAF, and confidence that laws won't change regarding donating to DAFs.
DW and I have no children. Some portion of our estate will go to relatives, and the majority to the DAF where we have set up a distribution plan. We change the plan from time to time as our charity preferences change.
Regarding #2, I think this is what you want. There are various rules at Vanguard Charitable. You have to donate 5% per year in aggregate. But in a good market, this could last a long, long time. Of course, DAFs do have yearly fees. I see this as paying for managing this quasi-endowment.
There are also a lot of rules as to what happens if the charities in your list go away. The percentages adjust among the remaining, and if you have none left, it goes to their general charity.
Please see more here:
https://www.vanguardcharitable.org/giving-with-vc/how-it-works/legacy-planning
One unique aspect of a donor-advised fund (DAF) account is the ability to continue charitable giving beyond your lifetime. A giving legacy, laid out in a succession plan, is an effective way to ensure assets in your account continue to fulfill your giving goals after you pass.