A step-child is considered a your child for tax purposes. You will indicate that they are your child, as long as you are eligible to claim them. Per the IRS: "Your son, daughter, adopted child1, stepchild, foster child or a descendent of any of them such as your grandchild" meets the relationships requirements to be claimed as a dependent. There are other requirements that need to be met and which are listed in the following TurboTax FAQ below:
https://ttlc.intuit.com/replies/5187060
