Nothing special to do. This is just a normal family record. Some people like to add “2 TYPE birth” to each child’s BIRT record. I don’t though, as I regard “birth’ as the default that requires no further explanation.
When you add the children to this family (using the “Family” page, ‘family.php’), select “Adopted” from the “Relationship to parents” box. This adds an “ADOPtion tag to the child’s record.
On the child’s individual page you can add an “ADOPtion event, selecting the appropriate FAMily record.
In both cases, you can also edit the adoption event to specify which (or both) parents adopted the child.
Nothing to do (in my opinion) as (you said) there is no adoption involved.