The standard protocol is to fill the odd line with available spots and the even line with available spots. You will be split into 2 lines when you get closer to the ride which is an odd and even line.
If you ask the Cast Member that is putting the 2 rows together, you might ask really nicely. Whenever I have asked at other rides, I have always been granted my own row since I'm usually a single traveler. With how new SDD is, I have yet to get my own row. If you explain it being your kid, I am sure the CM will let you. However, there are videos while in the queue that odd number parties will be placed with other parties. So, it does give the CM the decision to place your party with another party.
I don't know what the rules are, but I think I have seen the adult ride with another adult in another party and have the 2 kids ride in their own row. Not sure if that is an option or not.