UC child element after getting child benefit.

Hello everyone. I have posted on here about trying to get child benefit. My son is 8yo. He lives with me 6 nights per week, actually it turned to 7 since beginning of this year. Yesterday I got child benefit award. I'm sure his mother was claiming child element along with child benefit, and am assuming she hasn't reported change of circumstances. Despite school not wanting to change my address as 1st contact child benefit still gave me award. Do you guys know what is the process of getting 'child element if someone else is claiming it already?