Without this assessment families are unable to receive vital council support needed, warns disability charity Sense, which has carried out the research.
This involved a survey of more than 1,000 parents and carers of disabled children, and compiling assessment delay data from local authorities.
Among those surveyed just under half said that “they are struggling to cope” without support, said the charity.
A third of families who are granted support warn help from local services is “rarely available”.
Two in five parents who received support said they were only eventually successful in achieving this after having to take their case to a tribunal.
Even when seen, families face further delays receiving the outcome from assessments. Sense found that in 18 council areas the length of time for families to receive this decision is more than the 45-day legal limit.
The worst performing area is Bath and North East Somerset, where the average time it takes to decide is almost double this legal limit, at 88 days.
The next three worst performing local authorities are also in the South West of England, in Torbay, North Somerset and Somerset.
Other areas where parents are facing delays above the 45-day limit include Wigan, Nottingham, Shropshire and Blackpool.
The challenges parents face is making them anxious around continuing support, with nearly two in three fearing cuts will be made to their child’s care.
Parents are also pessimistic that the government will be able to improve the situation, with fewer than a third believing there is “genuine intent” from ministers to improve social care for disabled children.
“Access to high quality support can be life changing – it is the difference between a child feeling lonely and left behind, and a child having the support they need to grow, learn and thrive,” said Sense chief executive James Watson-O’Neil.
“But the social care system is failing disabled children and leaving families at breaking point.
“Parents shouldn’t have to fight for the care their children deserve. We need urgent reform to ensure all children get the support they need without facing unacceptable delays.
He is calling on the government “to make disabled children a priority, by establishing a clear and consistent pathway to support and a national threshold to assess all disabled children”. This needs to be backed by adequate funding, he added.
Case study - Carla Alderman and son Cameron
Among those impacted is Carla Alderman and her 12-year-old son Cameron, who is deafblind and missed out on support for several years.
This included vital early years support – despite the law saying a child is entitled to an assessment based on need, not age.
She applied four times for an assessment and was rejected for support when she was eventually seen.
Alderman said: “I wish the social worker had just come in and told me what I was entitled to.
"I didn’t know that carers could come into the home to help us with personal care. I didn’t know that there’s respite available. I wasn’t offered these things because they could see we were coping and thought I could just continue to do it all myself.
“I felt that if I wasn’t pursuing things and chasing people and contacting social care, Cameron would just be forgotten about."
When Cameron was seven years old, he eventually got support, including from intervenors, trained professionals who visit his home, paid for through direct payments
A more recent arrangement also allows Alderman to have a night away from home once a month, when a carer stays over to care for Cameron.
