To your first question, I'm going to say that you should be able to fit on everything with a 40" waist. Sometimes though people of different proportions have more difficulties than others even with the same weights or waist sizes. Hips are what will really make it difficult to fit if you carry a lot of weight there. If in doubt, Powderkeg and Outlaw Run still have test seats, and I think The Giant Swing may still have one as well. Thunderation is easier to fit in, so I think you'll be fine there in any case.
For smaller kids, there's a number of rides that should fit both of them and you at the same time. Other posters should correct me if I'm wrong or add on to this list, but just to name a few there's the galleon, tea party, fire wagon, fire chaser (balloons), happy frogs (I think?), all the water rides, and fire spotter (the little tower ride in FL). Then there's some rides like the GE coaster and waveswinger that you can all ride at the same time, but not necessarily right next to each other. Could put the kids into one car on the coaster with you right behind (the attendant is good about working these things out). The GE coaster is hard to fit into though, my brother couldn't fit and he isn't that big.
