What are the precautions for dental cleanings-prophylaxis without deep subgingival scaling of children with sickle cell anemia?  

No extra precautions. Routine dental cleanings (prophylaxis) for children with sickle cell anemia are not different than dental prophylaxis for normal children. Because children are not sedated, the risk of decreased oxygenation is not there. For "operative" dental procedures such as tooth extractions and root canals, prophylactic antibiotics can be considered.
No prophylaxis neede. No prophylaxis needed other than that your dentist would use for a routine cleaning with anyone in the pediatric age group.
No concerns. I am not aware of any concerns with prophy however deep cleaning could involve bleeding and or clotting concerns.