Having fast food once in a while is not an issue per say, the problem comes to children that life revolves around Fast Food.  Daily eating habits are important to not only your teeth but your overall health care.

Food that gets stuck in between teeth and gums can cause tooth decay and gum issues which can lead to pain.  Dental pain can affect their school life disturbing their concentration and hindering their learning capacity.

Eating too much fast food can result in obesity, diabetes and problems related to the heart.  Studies have shown that obesity and diabetes can cause larger dental issues with teeth and gums.  Bad teeth and gums in turn can also cause heart problems.

For young children still growing fast food normally lacks nutritional value and does not have enough calcium. Lack of calcium can cause harmful effects on new teeth coming in which could be unhealthy and lack strength.

This is not to say child can not have any  fast food,  but to balance out your child diet and make sure proper dental hygiene is being followed.