SIMPLE. If your kid/s call someone twice older by first name, and as a parent you allowed it - YOU are enabler. As part of our culture, Filipinos call older 'non-relatives' grandfather, grandmother, uncle or aunt, etc even when they are NOT actually related in this way. It is an acknowledgement of their age and respect. 

With that been said, I don’t think there’s justification that today’s generation can call people twice their age by their first name especially if they are your parents NOT by blood, but by choice. Imagine calling your Aunt/Tita or Uncle/Tito by their first name. That to me - is being ungrateful and disrespectful. 

Be a parent. Call out the bad behavior of your kid/s right away. There should be no next time. Excusing the negative choices and attitude of your kid/s, or dismissing that there is actually a problem - is not going to help them. Infact, you are enabling.