First I tried Joystiq Mii characters and in a short time made Wii-style avatars. I couldn't figure out how to save them but no worries: Windows 7's "Snipping" tool worked just fine. The results were quick and fun; not terribly detailed but o.k. if you want cartoony. I did run into a problem with gender and age differentiation; since there was only one body type I needed to use facial features. Putting eyelashes and long hair on the females, and a mustache on the daddy was an adequate tho not satisfying solution. In retrospect I might've used different lip styles too, since cartooning usually gives females fuller lips.
For age differentiation I tinkered with giving the children bigger eyes and smaller noses. Because you can move the features, growing or shrinking them, you can have a very large number of faces. I tried to give each child at least one feature directly from either parent. I would've liked more features, e.g. ears, body types, but for a quick cartoony avatar, I was happy.
I wanted a more realistics, "sexier" look, so next I when to
Yahoo Avatars. This definitely gave a more realistic (although still cartooned) look. One nice feature is that it saved your 10 favorite avatars; I still used the snip to save the avatar to my harddrive but it looked like I could keep up to 10 works-in-progress for tinkering. But a complication arose when I started playing with gender.
Gender differentiation was straightforward: you pick your gender and it gives you a female body type, and face/hair/clothing/accessory choices. However, switching gender was a problem: when you changed gender you lost all your favorites. It appears that yahoo's avatars are intended to be used for your login and not for other purposes. You might consider having a separate login for each gender (...or hey Yahoo! think about gender switching sometime!)
Age differentiation was also an issue. Yahoo avatars had a lot fewer face choices, since you can't move the features around as with the Wii above. To differentiate the dad from the son, I gave the former the face with a five-o-clock shadow, but I couldn't find something similar for facially differentiating mom and daughter. Instead, I had to do that with choice of costume, giving mom some definitely hotter outfits and the daughter garb that deemphasized body shape.
It would have been nice to be able to play with body proportions, for example giving the children relatively larger heads, as a way of suggesting age.
Since I'd met my goal, I stopped checking out avatar-building sites (but I'd like to get back to Simpsonize Me!, South Park Studio, and the interesting-looking Meez). None of these utilities are perfect, but for the price, what-the-heck!