Long Beach is, by far, the easier choice.
Nothing really to do in Commerce for your GF and 2 y/o, unfortunately. That said, the Crowne Plaza is attached to the hotel and the hotel/casino complex is right off the 5 freeway (a major N/S freeway). If she has a car, she should be able to get around to other tourist spots.
Here's a list of action at LA area rooms. I updated the Commerce info about two weeks ago (on a Thu night). Should be indicative...
LA Cardroom comparison chart
Hope this helps.
Edmond