From what I understand there is no hard-n-fast rule about inflatables in the pools. It seems to be at the discretion of the guards on-duty. A "general" rule of thumb seems to be, if the object blocks enough of the view of the pool bottom so the guards can't feel safe watching the pool, or the pool is very busy, then they may ask you not to use it.
However, I can't see your child's small float causing a problem in these departments.
You can also borrow a child's size PFD (life vest) from the Poly marina for free. Our youngest was almost three when we stayed at the Poly and that's what he used in the pool. It's got a crotch strap so the vest will stay on securely. He even went down the slide in it.