I'm also a single mom with 2 kids. Mine are older now, but once upon a time they were little. There are quite a few rides where you can fit 3 if you are relatively small. I'm not super skinny, but I fit with my boys when they were little.
On rides where you can't all fit together, you have to make some choices. You didn't say how old your children are. Most of the time I opted to let me boys ride together, and I would ride just behind them. This depends on the ride of course. You could also let your oldest ride alone, with you and the youngest behind. Only you know your kids and what you think they can handle.
There may be some that you have to skip until they are older. For example, Toy Story. You can only seat 2 people in each side of the car. So you would be completely separated from one or both of your kids, the seats backs are really high, you can't see anybody behind you. You may have to skip that one until they are older.
Your best bet may to be took online and get an idea of what rides you think you will want to do. Then go to you tube and check them out. You will be able to see how the cars are set up, and if you think it will work for your family.