How to give access to account team member so that they can access all related object with account?

Hi Audrey,
For this we use sharing via apex. In this you need to query account team member of each account with all the related object records on it one by one. And then create a share (Share is object in salesforce) for each child record by giving the team member id in its Userorgroupid field. Sample code can be found in below link.

Refer to this link:
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_bulk_sharing_creating_with_apex.html