Hi and welcome,
You need to create a family entity for each union.
In this case, F1 would have the first couple and children. You would add a MARR event and a DIV event to this first family.
Then for the parent who remarries, you create for him/her another family, F2. And you attach the spouse and the other children to this family F2.
I hope that clarifies.
Best regards,
Frederic