The NSPCC said 144 - almost a third of victims - had not reached secondary school age and 44 were five or under.
A total of 309 offences were reported against 11 to 17-year-olds.
The NSPCC is calling for action to reduce the number of sexual assaults on children with national figures revealing there were more than 400 offences reported to police every week between April 2010 and March 2011, but just one in 10 resulting in a successful prosecution.
Crimes include rape and incest.
Gordon Ratcliffe, regional head of service NSPCC Yorkshire and the Humber, said: “A concentrated effort has to be made if we are to start reducing this distressing level of offences, many of which are committed on extremely young and helpless children.”
Det Insp Mark Monteiro, of South Yorkshire Police, said: “South Yorkshire Police deal robustly with child sex offence allegations and look for solutions which are in the best interests of the child. Therefore, it’s not necessarily accurate to judge the outcomes of all these cases just on the number of convictions.”