Are you essentially asking whether you can buy e.g. two tickets for (non-existing) kids to ensure you get a full 4-sleeper compartment for just the two of you?

Yes, I have bought four tickets for three people to ensure we don't have anybody else in the compartment. However, do make sure that you get four tickets for the same compartment when buying them! I have, however, never even thought about buying a children's ticket, but bought an adult one. Interesting thought though

As the conductor usually makes a headcount or even collects your tickets when you settle into your compartment, there is the slight chance that he will try to put somebody willing to upgrade into your compartment. Small risk -particularily on crowded routes, but not impossible.