Ruth Neave, 26, was detained early yesterday morning and remained in custody last night as questioning continued.
Rikki's naked body was found, after a huge police search, in undergrowth in a copse only 500 yards from his home in Redmile Walk on the Welland Estate, Peterborough, on 30 November.
The day before he had played truant from Welland County Primary, and his mother reported him missing that evening.
The last confirmed sighting was at 6.30pm on 29 November outside a row of shops with a group of children. Eighteen hours later his body was found and 150 yards away his clothes were found in a dustbin. A post mortem examination showed he had been strangled, but not sexually assaulted.
Neighbours described the chubby schoolboy, one of four children, as "a lovable rogue". More than 100 officers were assigned to the case and last month two teenagers were arrested and questioned about the murder. They were released without charge.