S.V.
They sell a yellow bath sponge that won't move while submerged in the big tub, I've been using that for my almost 1 yo for the last 4 months. They have them at Target for a few dollars. The other thing I've used is a rubber bath mat. I found one at Walmart that is long enough to cover the length of the tub and has suction cups to stay in place. I used that for my son when he was a little older than 1 until he was about 2. That might be better if he likes to stand up in the tub. That's how my son was and he wouldn't slip. It's also machine washable. Good luck.