I see quite a lot of polyurethane on wood, and I nearly always perform the same procedure on it. Outgassing at this point would be minimal, but polyurethane looks like what it is -- plastic -- and it can be scratched and ingested by little ones.
We sand the floor with a fine screen and a trailer-mounted HEPA vacuum extracts the dust. This removes most of the polyurethane from the surface, but not out of the wood. Then we polish the surface with a good hard-paste wax and a floor buffer. This brings back the luster and shine of the wood, and it gives you that nice old-fashioned look that you probably were going for in the first place. Wax does require maintenance. Bioshield has a nice product called Floor Milk that will clean and polish but not build up. They also carry a nice floor wax for the initial application.