An unpublished report to the Welsh Assembly by the All Wales Network of Children's Occupational Therapists (COT) said an extra 97 paediatric therapists were needed "to bring caseloads down within recommended levels and meet the needs of children on existing waiting lists".
The report - excerpts of which have been seen by TES Cymru - said therapists were dealing with 35 to 120 cases each instead of the recommended caseload of 30.
It also called on the Assembly government to carry out a comprehensive review of occupational therapy services for children.
The lack of paediatric therapists has created huge delays in treating children with poor motor skills. A COT source said that while those with more severe conditions, such as cerebral palsy, were generally seen within 15 weeks, many children have to wait for up to two years.
The mother of one statemented nine-year-old, with Asperger's syndrome and poor fine motor skills, has been waiting five months for a first appointment in Newport.
"We know there are probably many others more needy than us waiting," said the mother, who asked to remain anonymous.
"However, if there was more provision, our son could get the help he needs sooner. That would make a huge difference to his progress in school."
A spokesman for Gwent NHS Trust said it has 76 children waiting for assessment in Newport.
"Young babies with cerebral palsy and older children with disabling conditions get seen first. Our ability to see others depends on how many high-priority cases are referred in any given month. Children in less urgent categories can wait up to 16 months."
Practitioners say delays in assessment undermine the goal of early intervention, considered crucial to successful therapy.
Karina Baudinette, specialist OT and head of clinical services at the Dyscovery Centre, Cardiff, which provides advice, training and assessments related to dyspraxia and other learning difficulties, said: "Many cases can be very damaging to learning, but they do not require long treatment.
"With the right advice families can ensure their child progresses. But if help is not there, it can result in low self-esteem, poor behaviour and underachievement."