Manchester City Council said it will use the cash to employ additional social workers and team managers, with the aim of lowering social worker caseloads from the current average of 24 to 18. It aims to employ 132 members of staff by 2021.
Ofsted rated children's services in Manchester "inadequate" in 2014. Inspectors said nearly 500 children were potentially left at risk because social work assessments were not carried out quickly enough.
The council has invested a total of £14m since then to improve children's services. Other improvements include recruiting a new senior leadership team and permanent director of children's services.
Paul Marshall, director of children's services at the council since March this year, said that social workers do not have the capacity to do as much as they could if they had smaller caseloads.
"Now there is also a stable and permanent leadership team in place, the timing is right to invest this additional money directly in social work staff over the next five years as part of our longer term strategy to reduce demand and sustain the improvements already made - so that services for children in the city are as effective and efficient as they need to be," he said.
"Our clear gaps at the moment are in the number of social workers we employ. In order to consolidate the improvements already made and to continue to make further progress, we must reduce social worker caseloads even further.
"A reduction in the number of caseloads each social worker has will mean that in the future outstanding social work practice will become the norm and not the exception - and that is exactly what we all want to achieve.
"More social workers, managing smaller caseloads, and working to the highest possible social work standards."
