Answer: I think you mean DPA (Data Protection Agreement/Addendum) and not DPIA (Data Protection Impact Assessment) and the answer is no.
2. Does the GDPR supersede with the national "Data Protection Policy" since the company is dealing with individuals in EU?
Answer: If the GDPR is applicable to your company, then you should comply with its requirements at least for the processing activities that are touched by the GDPR. For example, if you also have clusters that are not in the Union, then the processing activities related to them should not necessarily be compliant.
3. If a company has a foundation policy for children, does the responsible party/parent need to give written consent?
Answer: In the case of minors, consent should be obtained from their parents or legal guardians.