Available stats seem to only go up to 2003 -
missing persons helpline website says
"Home Office (2002) figures show that the number of homicides by children involving children under 16 has remained constant over the last 20 years, with about 7 occurring each year".
However it also shows police recorded offences of child abduction (by anybody, so parental ones included) were:
2000/2001 546
2001/2002 583
2002/2003 846
"Abduction by a stranger or non-family member is rare. Whilst the police record several hundred offences of child abduction every year...(including parent abduction) there are very few offences which involve the abduction and murder of a child by a stranger."