Well, you can probably assume highly cryptic colouring and a deeply skittish nature,
but consider sea turtles --
Yes, but they are armored and can swim at reasonable speeds.
Also note that the whip-tail sauropods and mace-tail sauropods probably have these defensive measures from a small size, and that they'd apply to equivalently sized predators just fine. So Ornithosteles is fine with the hatchlings, and maybe even the yearlings, but has some serious cost-benefit concerns thereafter.