It depends what the divorce decree states. In many cases, one parent may be ordered to provide the children with insurance, and the parents may split the cost of the insurance or one parent may be solely responsible. For any medical expenses not covered by insurance, one parent may be responsible or the expenses may be split.
If the divorce decree does not specify who is to pay the expenses, they may be considered the responsibility of the primary custodial parent. However, you can always ask the court to have your court order modified so that you two share the costs.
In most cases, medical and dental expenses are separate from child support payments. That means that any amounts spent on medical and dental expenses are the responsibility of one or both parents. If you have encountered medical expenses for your child that your ex will not pay, you may need to file a petition with the court for enforcement in order to get the money you are owed.
In some situations, if the medical treatment is ongoing, such as braces, you may consider asking your ex to pay for his or her share of the bill directly to the provider. This will decrease the amount of interaction between the two of you, and it may decrease any hostility if a check is written to a doctor or dentist rather than to the ex.