I have thought of this before, and thought of it from all angles. The conclusion I have come to is that all the races (besides Kran and perhaps the Lemur) must have very closely related gestation and maturing rates. That means they could very greatly, as some humans grow and mature at differant rates.
Reason one: A Fenki can have a Ylian baby, or any other race's child for that matter. This means the gestation would have to be within a few weeks of each other either way, or the babies would not survive well. If a 'human' baby was born to a Fenki in six months, as RPed, the baby would be very bad off. Sure, you -could- claim the mother's carrying time would change depending on type of race being carried, but I don't really see that as a realistic option.
Reason two: The races were not chosen at random to come through the Portals. Vodul hand picked the races he wanted to become a part of the world. Since he was doing so as a favor to his underlings (the other gods) he would not have picked races that had completely diferent features in all areas. Being able to have cross race children is a major example of this. Growing up faster is actually a bigger issue than looking different, in my mind.
Reason three. The Enki are not cats any more than Ylian are human. The may -look- like cats in some features, but are far from. There is no reason to assume they would have any more cat-like behavior or physical growth relation to Earth cats than the Diaboli would. In truth, from what I understand of the Enki, they act almost nothing like Earth cats (small or large) and more like human tribes (such as American Indians), or in the animal kingdom, wolves. They are even called 'packs', not 'prides'. So, don't judge a book by its cover, or a race by its skin.
The best bet is to hold your RP as close to 'human' as you can, with little race tweaks to make it interesting.
![Smiley :)](https://www.hydlaaplaza.com/PlaneShift/smf/Smileys/custom1/smiley.gif)
As to gamemechanics-wise, I have asked Talad before if there are to be child characters/models in the game, and he has told me there are no plans for it. Sad.
![Sad :(](https://www.hydlaaplaza.com/PlaneShift/smf/Smileys/custom1/sad.gif)
*edit: RP options if you wish to play the child as more mature:
A time twist portal or some such to a plane where time travels faster. It worked in Angel.
A potion of fast growing up +16.