const regex = new RegExp('^a[0-9]+-[a-zA-Z]+$');
const parentElement = document.querySelector('#parent-element');
const items = parentElement.querySelectorAll(`[id]`);
const children = Array.from(items).filter(item => regex.test(item.id));

console.log(children); 

<div id="parent-element">
  <p id="a1-a">Child 1</p>
  <p id="a1-b">Child 2</p>
  <p id="INVALID-1">Child 3</p>
  <p id="a10-f">Child 4</p>
  <p id="INVALID-2">Child 5</p>
  <p id="b1-a">Child 6</p>
  <p id="a1-2">Child 7</p>
</div>