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The process for obtaining child keys from Firebase Realtime DB using pagination involves the following steps:

1. Determine the total number of child keys in the database.
2. Determine the page size - i.e. how many child keys you want to retrieve at once.
3. Calculate the number of pages by dividing the total number of child keys by the page size.
4. Retrieve the child keys for the desired page by using the startAt() and endAt() methods.
5. Save the last child key retrieved and use it as the starting point for the next page.
6. Repeat steps 4 and 5 until all child keys have been retrieved.

Here's an implementation in JavaScript:

const databaseRef = firebase.database().ref('path/to/child/keys');
const pageSize = 10;
let lastKey = null;

databaseRef.once('value', snapshot => {
  const childKeys = Object.keys(snapshot.val());
  const totalPages = Math.ceil(childKeys.length / pageSize);

  for (let i = 0; i < totalPages; i++) {
    const startKey = lastKey ? childKeys[childKeys.indexOf(lastKey) + 1] : childKeys[0];
    const endKey = childKeys[childKeys.indexOf(startKey) + pageSize] || childKeys[childKeys.length - 1];

    const pageRef = databaseRef.orderByKey().startAt(startKey).endAt(endKey);
    pageRef.once('value', pageSnapshot => {
      const pageChildKeys = Object.keys(pageSnapshot.val());
      // do something with the child keys retrieved for this page
      lastKey = pageChildKeys[pageChildKeys.length - 1];
    });
  }
});