Short answer: It must be available, though you are not required to buy it. Dental coverage for children 18 and under is one of the ACA’s essential health benefits, so when you shop for coverage for a child, a dental option must be offered, either built into a health plan or as a separate dental plan. Adult dental is not an essential health benefit.
The essential health benefits include pediatric services, including oral and vision care, so children’s dental coverage has to be available to families buying coverage. Dental coverage is an essential health benefit for children, which means that if you are getting health coverage for someone 18 or younger, a dental option must be offered. You do not, however, have to purchase it.
For adults, dental is different: it is not an essential health benefit, so health plans are not required to include or offer it. Many people buy a separate stand-alone dental plan for adult coverage. Note that stand-alone dental plans can have waiting periods before adult services are covered, so check the plan details before enrolling.