Can I Have 20 Amp Receptacles Downstream From A 30 Amp Receptacle?

Suppose I have a 30 amp producing generator (has 30 amp breaker on the generator itself). I then run a 10-2 wire from the generator to a 30 amp outlet. From there it splits off to three 20 amp outlets with 12-2 wire. Would it be appropriate to run the 12-2 wire from the 30 amp outlet. I’m thinking the only current flowing will be that which is pulled by the device. If so, the 12 gauge wire and 20 amp receptacles are adequate since the devices on those receptacles will only be pulling up to 20 amps. If the 20 amp receptacles are not used, than the 30 amp receptacle will be used, and vise versa. If the 30 amp receptacle is used, than the other three outlets get left alone. In other words, I will be using the 30 amp OR the others. The 20 amp outlets OR the 30 amp outlet. Is my thinking correct here?