Medical negigence claim period

The negligence medical claims must be commenced within three years, this called the limitation Period of claim, that period sometime extended also but only in some cases like if patient does have mental capacity to bring a claim then limitation will not start until their recover. But need to keep all the documents. In the case of a child and a patient, a claim can be pursued by someone else acting on their behalf. This role is known as being a Litigation Friend and may be fulfilled by a parent, in the case of a child, or a spouse or partner in the case of a patient.
I have one question here, in child case how time will be calculated for claim process, because in this age it is difficult to find out the exact cause of the problem and how his/her life will be affected after negligence by medical.