A small amount of substrate isn't a problem. Just try to minimize substrate ingestion.

I've had one of mine go 8 weeks when he was 6 months old, so I wouldn't worry too much about it. Just make sure that the temperatures/humidity levels are good and you offer a rodent that weight 15% of the snakes body weight every 7 days (every 4 or 5 days if she is less than a year old)