Why not just use the "stretch skeleton" cheat to "adjust" the height of the child/teen to taller or shorter?  Or making them a bit more chubby in body/face by using "change appearance" for that in-between child and puberty age?  You can then make them thinner and less full in the face when they mature....  just a thought... 

(Or am I missing the point somehow- which knowing me- is totally possible!)
THIS is a gripe of mine as well...
I'd LOVE to see new age groups for the Sims!!! 

How ideal would it be to have:
Baby to Toddler
Toddler to Preschooler
Preschooler to Child
Child to Preteen
Preteen to Teen
Teen to Young Adult
WHOO HOO
I'd be a happy camper with this much "realism" added to my sim-world...
I really hate that there's no "in-between" ages for the kiddo's.  

*hello Maxis are you listening?

*
LOL
ANYhoo... now I'm rambling and in danger of hijacking your thread... so I'm going to shut-up now. 
Hope you find something that works for you. 
