The toddler who was at Fairy Tales Nursery, in Dudley, was found suffering critical injuries on December 9.
Paramedics were called to the now-closed nursery and the child was taken on life support to hospital where he tragically died.
A criminal probe was launched after Ofsted inspectors closed the award-winning nursery in the wake of the death.
Three women, aged 20, 23 and 50, were arrested on December 16 on suspicion of gross negligence manslaughter and subsequently released on bail with strict conditions while enquiries continue.
A further three women, aged 51, 53 and 37, were arrested on Wednesday two were on suspicion of corporate manslaughter and one on suspicion of gross negligence manslaughter.
Fairytales Day Nursery was closed over concerns children were ‘at risk of harm’, according to the Ofsted report.
The small employer of the year award-winning nursery must now take action to avoid being closed for good.
A post-mortem examination into the one-year-old’s death has already taken place, police said, but more tests are needed to establish exactly how the boy died.
Family members are being supported by specially trained officers, the force added.
A West Midlands Police spokesman said: “We are treating the death of a one-year-old boy on 9 December at a Dudley nursery as suspicious and an investigation is under way.
“The criminal investigation was launched following a visit by Ofsted after the boy’s death. The nursery, in Bourne Street, Dudley, has been closed by Ofsted along with other linked premises.”