Sparse (pediatric) and rich (adult) data

Hi, I am working on a pop PK model to estimate PK parameters in pediatric and adult patients. Pediatric study (n=20, age <6 yrs) has fewer samples (3) per subject whereas the adult study (n=50, median age 20 yrs) has 12 samples per subject. A two-compartment model best describes the data for each data set. Although a two-compartment model best describes the combined data, the individual parameter estimates in pediatric population are different compared to those obtained using with pediatric data alone. Note that the parameter estimates in adults were not significantly altered with either combined or adult data alone. Body weight is the only covariate included in the model with allometric exponents fixed to 0.75 on CL and 1 on V1. I would like to hear your thoughts on this and any suggestions on how to proceed with modeling combined data from pediatric and adult studies. Regards, - Chandra