AXYZ Studio has low-poly, but good looking child figures - for MAX, basically. Doesn't require Poser - does require that you have MAX, Maya, Lightwave, Cinema4D  or some other program allowing you to "rig" and pose figures in "T-form." (at this link, and scroll down to the fourth item.)

http://www.axyz-design.com/axyz-design-3d-humans-characters-metropoly-tpose.php

They also have some that are already rigged,for just a little bit more money that are at the link shown below. These are well-worth the little bit of additional money if you have one of the 3d modelling programs (listed above) that will let you use a rigged figure. That is, rigging a figure usually requires several days to get it right. So, the difference in price between $94 USD and $118 USD is easily worth it.

AXYZ Studios also sells a very large array of "motion files" at nominal charge that can be used with their rigged figures. That is, if you have access to one of these higher end 3d modelling programs, you can take their rigged child figures, apply one of the motion files, and have a natural looking child in play in minutes.