Paediatricians are left having to guess child dosages as a percentage of the adult dose, and some medicines which are given to children - such as vitamin K, given to babies at birth - are not licensed for humans.
One medicine used on premature babies to help them breathe has never been approved and is not intended for children under three.
The Commons Select Committee on Health, chaired by former Conservative minister Marion Roe, said it was "astonished" to discover that medicines tested and licensed for adults were given to children without proper checks. The MPs said the public would also be astonished at the discovery.
Many medicines currently given to the children are administrated by a route, in a formulation, or in a dosage which has not been approved by the Medicines Control Agency. The Government has been aware of the lack of checks on children's medicines since a working party report in December 1995.
Department of Health officials said the situation was "clearly unsatisfactory", but pointed out that the Medicines Control Agency had no evidence that any children had suffered harm as a result of a lack of specific testing and licensing for children.
Expressing deep concern, the MPs said: "In the absence of a satisfactory system of testing and licensing, we cannot know that this is the case, nor can we know of the minority of cases - possibly a very tiny minority - where children have suffered actual harm from taking such medicines."
Audrey Wise, a Labour member of the committee said the MPs did not want parents to panic. "There is this `guestimate' attitude and I don't think we would stand for that with adults," she added.
Experts in child health said parents should not fear their children were at risk. The most commonly used medicines such as cough mixtures and antibiotics are fully tested on children before being sold over the counter or prescribed.
The committee has spent months investigating children's health, and said it did not produce a more comprehensive report because of the shortage of time before the general election. But its findings will put children's health high on the agenda for whichever party is returned to power.
It uncovered a catalogue of neglect of children's issues and a failure to implement recommendations going back to the 1950s, including the lack of a central performance monitoring system for children's health services and the failure to provide a central framework to ensure local inter-agency co-operation.Reuse content