Do you know if she has already received the chicken pox vaccination? If she has, she can still get chicken pox, but usually it won't be as bad. Also, keep this in mind.
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You stated you got sidetracked with other issues during the visit and forgot about the rash. Apparently it isn't so bad that the child complained much, or you would have remembered about it. This may also be happening with the mom.
If you called social services, they would ask you why you didn't take the child during this past weekend if you were so concerned.
Your best bet is to follow up with the cp and ask if the rash has improved or worsened. If the child has chicken pox, a trip to the doctor is not necessary unless he/she begins to run a high fever, over 104. with it. Calamine lotion for chicken pox or bug bites works well.
If she is in school, go visit her next week during school, at the school, and ask her about her rash. Perhaps have the school nurse check her over.
That is the best advice I can give, perhaps someone else has other thoughts.