@Cleotheneo,
Hmm... looks like you need to start with a limit. I'd take that to mean something that would apply to both, like "go to bed on time." (Or if it needs to be expressed in the negative, "don't stay up too late.")
How would you express that limit to each child?
I think once you figure out the limit, the rest of it will come pretty naturally from what you've read. If not the bedtime one, another one that would apply to both ages.