On the exterior bottom of the tub, there is a non-skid vinyl that prevents the tub from sliding around. (earlymomentsmatter.org)
Anti-slip bathtub treads on the bottom of the tub like these adhesive anti-slip circles from Puj help family members keep their traction. (thefashionablehousewife.com)
For kids who can sit up, a bath ring suction - cupped to the bottom of the tub may provide you with an extra «hand.» (babycenter.com)