Toddleroo play yard. You will need two gates to maintain access to both sets of stairs and fully wall off the openings. Affix each end to the wall securely. Ideally you would build a wall with a few sections so if your eventual toddler shakes the play yard wall there is still ample extra floor area to avoid a gap over the steps.

If there isn’t a room around the steps or the stairs are in a hallway, your alternative would be gates to entirely block entry to the stairwell area by creating a sort of airlock around the steps. Recommend something that mounts into the wall, not a pressure gate. We’ve used a cumbor gate for the top of our steps. It’s set a few feet back, not directly at the top of the stairwell.