OP here, thank you everyone for you thoughtful responses, advises, and sharing your personal experience/plan on this topic:
While I totally agree with letting the kids to have skin in the game and it is a great lesson for them to manage their finances. However, it makes me feel good to help out. I learn this from my mom who was a single mom and had scarified all her life for us to have a better life then her. Happy mother days to my late mom and all the moms out there.
As mentioned, I've told my children that I will fund their 4 yrs (in state) colleges and they will be on their own after 4 years if they decide to continue. (In oldest child case, it will be ~$200K in loans). I plan to take half of the college funds of the 2 younger kids and loan it to the 2 olders, so the older ones will have part of their loans interest free.
Obviously, the risk is that if they won't finish their grad school and/or won't pay back the personal loans. And to provide an insurance for this risk I can do OMY or 2MY as some has suggested. Beside I would like to have some paddings to my FIRE plan, I feel kind of nervous to pull the trigger when none of the kids are yet to launch.
Thanks