Are children responsible for parents medical bills?

No, but bear in mind that the costs will come out of the parents' estate first.

So if a single parent died with assets worth $12,000US, the medical bills will be paid after the necessary amount of assets have been liquidated. The child will then inherit the rest. If the medical bills are higher than the deceased's estate, then all of it will be taken, but the rest of the debt will be written off.

Note also that many countries have laws that consider it an attempt at fraud, for a terminally ill patient to sign all their assets over to someone else before death, to avoid the patient's debts being paid out of their estate, post-mortem.