Sir Paul Ennals, chief executive of the National Children's Bureau charity, said it was unclear which aspects of child health GP consortia would be responsible for.
'Most GP consortia I speak to are really worried about how to fulfill their role, particularly in child and adolescent mental health and services for disabled children,' he said.
'Most child health services really need to be commissioned alongside other children's services,' he told GP's sister publication, Children and Young People Now.
GPC deputy chairman Dr Richard Vautrey said: 'There is no document to explain how children's services will be commissioned, but that doesn't just apply to children's services. At this stage in the reforms there is very little detail on commissioning in any area.'
To commission children's services more effectively, GPs will need to work more closely with their local authority, something which many consortia have already begun to do, Dr Vautrey said.
Health visitor services will be commissioned by local authorities under the NHS reforms, making closer working between GPs and local authorities 'inevitable', he added.
GPC negotiator Dr Chaand Nagpaul said GPs would seek advice from managers, nurses, hospital doctors and social workers in commissioning child health services.
'There is a need to move away from the idea that all commissioning will be done individually by GPs,' he said.
The DoH recently issued guidance for commissioners providing services for young people, entitled Quality criteria for young people friendly health services.