It is my understanding that the OP's kids have been under home quarantine for weeks and both parents recently had covid, and are now consequently most likely immune for a while. (Kids may have had covid as well? Unclear if they were ever tested?) Anyway, seems entirely reasonable to me that they would allow these specific girls into a different cohort for a few practices. The chances of these girls being infectious is extremely low. Plus they probably have several other mitigations in place like distancing, symptom checks etc. If my kid was at gym practice with these girls, this decision to let them practice outside thier cohort would not bother me in the least. Seems clear to me they are the least likely source of infection.
If my middle child's school ever reopens, his class will be split into two cohorts attending different days, but with the same teacher teaching both cohorts in person. Explain that logic if you can.