Children are the responsibility of not one parent, but both.
❶Children belong to not one parent, but both.
(子供は両親一方のものではなく、両者のものです)。
❷Children are the responsibility of not one parent, but both.
(子供は両親一方の責任ではなく、両者の責任です)。
Responsibility-責任
例えば:
Both parents need to take equal responsibility in raising their children.
(子育ての責任は両親がともに、半分ずつ負うべきです)。
参考までに!