A few months ago read that brain tumours&cell phones can be linked? why let kids have them?

Brain tumor: The link between brain tumor and cell phones is anecdotal at best and has not been proven in the scientific literature. As far as we know, cell phones are safe. It is a good idea to teach kids not to use them while driving. Distracted drivers have a much higher accident rate.

While there are: Purportedly inks to "radiation" from cell phones and brain tumors, if there was a link, one would expect and astronomical increase in brain tumors from 1990 to 2010. There is no change. So if there is a link, it is weak and causation is quite inefficient and low. One has to balance the risk (low) with benefit, and that depends on the child, their age, and maturity.