No, as others have stated CM's can't watch your son.
Your older child does meet Disney's requirement to ride alone (7 or older) so if your comfortable walking through the line and taking the chicken exit with your 7yo your 8yo could ride alone. Don't know for sure if I would do it however! The catch is that the 7yo must be tall enough to ride even just to walk through the standby, no exceptions.
At least that way it will be your 8yo's choice to ride or not and no blame goes to the youngest! My girls were 7 and 9 on the last visit and I gave my 9yo the option of riding RnRc alone last trip and she decided against it. FWIW she has done it many times before and I would have walked her to the loading area, taken the chicken exit and met her at the finish.
Have a great trip!
TJ