To use kid model with SMPL-X code, you need to download the kid template vertices from AGORA Download page.
Then you can use the smplx api just by adding the flag age=='kid' and providing the path where you stored the downloaded kid template vertices in flag kid_template_path.
import smplx
model_smplx_kid = smplx.create(modelFolder, model_type='smplx', age='kid', kid_template_path=smplx_kid_template_path)
model_smpl_kid = smplx.create(modelFolder, model_type='smpl', age='kid', kid_template_path=smpl_kid_template_path)
If num_betas is set to n it will create a model with n+1 betas where the n+1th shape component is the interpolation of SMPL/SMPL-X and SMIL/SMIL-X template. Note that it is recommended to use gender= 'male' for male kids and gender='neutral' for female kids.