Catherine, I don't know which insurance company you are going with, but they are pulling a fast one on you. Under the ACA, if you are getting a plan that covers a young person(kid) then child dental must be AVAILABLE to this policy, but you still DON'T HAVE TO BUY IT!

Here is the link and a excerpt from it:

https://www.healthcare.gov/can-i-get...e-marketplace/

"Adult and child dental insurance in the Marketplace

Under the health care law, dental insurance is treated differently for adults and children 18 and under.

Dental coverage for children is an essential health benefit. This means if you’re getting coverage for someone 18 or younger, dental coverage must be available as part of a health plan or as a stand-alone plan. While it must be available to you, you do not have to buy it.

This is not the case for adults. Insurers don’t have to offer adult dental coverage.

Under the health care law, most people must have health coverage or pay a penalty. But this isn’t true for dental coverage. You don’t need to have dental coverage, even for children, to avoid the penalty."

Never never never take an insurance companies word for anything. Never! This also goes for clinics/hospitals who send you bills after usage. First go to your insurance webpage and check dates and claims. Chances are your insurance company has paid already and the clinic/hospital "accidentally" sent the bill before they realized it.
Revisit that insurance company, read them the quote from the healthcard.gov site, then change companies! And tell them why. Don't do business with a company who is trying to fool you into buying something you don't need.

Oh, and no, the ACA isn't confusing or bad or communist-come-knocking at the door! (sorry Alan, foiled again!) Bookmark the site and use it. It really isn't complicated.