One primary adult account with 2 headsets both for kids

Yes, but there is a big caveat. You can only share games from your account to one headset at a time using App Sharing. So if your children want to play a particular purchasable multiplayer game together, you'll still have to buy 2 copies of the game (the second copy, I think, will need to be "gifted" to the other child's account during checkout).

You can add more family members from the Family Center either in the mobile app or on the web (I recommend using the web, as the phone app sometimes likes to say "something went wrong"). After you have set up both your children's headsets with your primary account, and assuming you have successfully added both children to Family Center, you can now use the phone app to "invite" the child to one of the headsets (or maybe that already happened when you created the child's profile in Family Center? I forget the exact steps now). You can select which headset by clicking the small chevron to the right of the device in the phone app (it is rather inconspicuous and easy to miss). It will bring up a list of the devices you own. Then select the right headset to use.

Then choose "Manage profiles->Add profile. Now, instead of directly adding their profile here (most likely it will "go wrong"), choose to send an invite to an email address. Put in your own email and use the link it sends you to open your web browser (preferably do this on a PC if possible). Then follow the instructions on the web page. Then repeat the process for the other headset and other child.