In terms of return flights to and from the US they can be as cheap as $800 and as much as $2000.
In terms of flights from LA to Orlando, around $200-$300 return from memory....Have a look on Google Flights to compare (Though Southwest Airlines doesn't seem to show up)
You can get direct LA to Orlando flights, or have a short 1hr connection somewhere like Dallas, which isn't too much of an odd routing.
In terms of parks, LA has 5 majors, and it depends on what your interests are.
Disneyland and Disney California Adventure are both must dos. A 3 day park hopper should allow you to see everything you want to.
Just up the road is Knotts Berry Farm, with a good mix of thrill and family rides and a couple of nice dark rides. A day is enough.
Universal Studios is definitely a must do ..The studio tour of Course, but the Harry Potter and Transformers rides are worth the price alone.A day is enough, and you can fork out for a VIP pass to skip the lines, but I think it's unnecessary.
Finally, Six Flags Magic Mountain is around 100km north of Disneyland, and has the biggest collection of roller coasters in the world, with just about every type represented.....flying, flipping, floorless, stand up, launched, racing, wooden....If you like rides like DC Rivals and Superman Escape, youll get something out of Magic Mountain (But the 6 yo might not)
One day is enough, but pay extra and get a Gold or better flash pass, which lets you jump the lines, and is probably worth it for this park.
Going in September is past the peak, but all Parks in LA and Orlando will be open daily.
Theme parks In other parts of the country they might only be open weekends.