The Bicycle Helmet Initiative Trust (BHIT) - a body campaigning for mandatory cycle helmets in the UK- is prone to exaggerate both the risk of head injury (especially to children) and the benefit provided by cycle helmets. The statements below have been attributed to BHIT and are examined against the evidence that is available.
From 2000 to 2002 the average number of child cyclist deaths per annum was 22 of whom the number killed due to head injuries averaged 12 . Both of these figures are very much less than the 50+ suggested. Furthermore, both child cyclist deaths and head injuries are declining although child helmet use in the UK is also falling.
In 2002, the number of serious child cycling head injuries admitted to A&E departments in England was approx 400 . Pro-rata population, that would be around 470 for the UK. This equates to an average of 9 per week. Most of these children made a full recovery.
Over 70% of all impact deaths involve head injury. In fact, one study  has shown that of those who die in crashes, cyclists are a little less likely to have suffer head injury than pedestrians and motor vehicle occupants (82% and 86% respectively). Most cyclist fatalities involve multiple injuries and very often the elimination of a head injury would be insufficient to prevent death from occuring.
From 2000 to 2002, the average number of child cyclists who died from head injuries was 12 out of an average total of 22 deaths per annum . This represents a head injury factor of 55%.
In 2002, cycling was implicated in only 7.1% of all child head injury admissions in England . Similar data from Australia, pre-law, shows that child cyclists account for only 8% of child head injuries from all causes. 
There is no known authoritative source for the figure of 100 000 injuries.
In 2002, there were 5,804 child hospital admissions in England associated with cycling (6,965 pro-rata population for UK). Of these, 2,183 (2,620 UK) involved head injury . The proportion of head injuries, 37.6%, is lower than for child pedestrians (43.7%) and only a little above the average for all child admissions (34.2%).
In England in 2002 there were approx 400 serious head injuries to child cyclists and 10 deaths . Pro-rata, the total number of such tragedies in the UK was about 470. Of these, one-third were associated with parts of the head for which helmets offer no protection. Other casualties involved multiple serious injuries, and an unknown number of the remaining children wore helmets. Even if helmets were 100% effective, fewer than 314 tragedies (2/3 of 470) could be avoided.
There is no real-world evidence that cycle helmets are effective in preventing serious or fatal injuries  . However if they were, the total costs saved in preventing 12 fatalities and 300 serious injuries would be £47,867,880 .
The minimum age for a full driving licence did not change anywhere in Australia and remains at 18. The age for an accompanied learner licence was lowered, from 17 to 16, in only one Australian state - Victoria in July 1990. In all other states there was no change at all.
In Victoria at the time the helmet law was introduced, 16 year olds accounted for 9.9% of all traffic . Even if every cycling 16-year old had changed over completely from cycling to driving accompanied by another person, that would account for only a small part of the 43% drop in cycling by Victorian teenagers that followed the law. Nor would it account at all for the decline of up to 60% in cycling amongst teenagers seen in other Australian states.
The energy released on impact increases with the square of the velocity, according to the formula E = mv2/2. This means that a helmet capable of maintaining its structure at the higher speed would only reduce a 30 mph impact to one of 27.5 mph, which would make very little difference to injury sustained.
However, cycle helmets fail catastrophically, not gradually, and usually at an impact velocity not much greater than the design velocity. After a helmet has been fully compressed and breaks, it offers no further protection to its user and passes all residual energy to the skull. Very few helmets offer useful protection in crashes where the impact velocity is above 12 mph. Indeed, a substantial proportion of helmets subjected to independent tests failed well short of their design velocity and would offer very little protection at all in real-world crash scenarios. 
Before advocating any safety intervention, the possibility of harmful side-effects must be considered and every possibility of this acknowledged and thoroughly investigated. In the case of cycle helmets, there is strong prima facie evidence from Australia and Canada that enforced helmet laws have increased risk for those who continue to cycle .
More widely, it is very common to come across claims from helmet wearers that their helmet has protected them from serious injury or death, to an extent that is out of all proportion to the actual head injuries suffered by bareheaded riders . This suggests that helmet wearers may be more likely to crash or to hit their heads if they do so.
These possibilities have never been the subject of research, and until that is done it is not possible to say with any certainty at all that there is no downside to wearing helmets.
Furthermore, it is now widely accepted that helmet laws (and most likely, too, helmet promotion) deter people in large numbers from cycling and that the falls in cycle use are long-term, especially among children and for utility journeys . As the health benefits of cycling greatly outweigh the risk of injury, many people who are discouraged from cycling by helmets suffer worse health and may even die prematurely as a consequence.
 Statistics from UK Department of Transport for on-road fatalities and Leisure Accident Surveillance System for off-road. In Australia in 1988, before helmet laws and when only a small proportion cyclists wore helmets, head injury was listed as the cause of death in 46% of cyclist fatalities (43% for over 13s, 58% for children under 13).
 Victorian Bicycle Strategy, 1990.
 Analysis of child hospital admissions data, 1995 - 2003, using data provided by the Department of Health for England.
see: How common is head injury when cycling?
 see: What evidence is there that cycle helmets save lives?
 see: What evidence is there that cycle helmets reduce serious injury?
 Costs from Highways Economic Note No 1, 1998. Department for Transport, UK. Healthcare costs are only a part of the total costs quoted.
 see: Location of injuries to cyclists.
 Kennedy A. The pattern of injury in fatal pedal cycle accidents and the possible benefits of cycle helmets. British Journal of Sports Medicine, 1996 Jun;30(2):130-3.
 see: Heads up - the science of helmets.
 see: Helmet laws: what has been their effect?
 see: A helmet saved my life!
 see: How helmet promotion and laws affect cycle use