Are teeth whitening kits safe to use for children?

Cost benefit: There is evidence that teeth whitening may harm the enamel very slightly. Sensitivity can be a symptom experienced with teeth whitening. Overall, the damage is very very minimal when compared to the esthetic outcomes. So, i'd say they are pretty safe.

Mostly yes: Most of them are but you need to read the labels to see what the concentration of the bleaching agent is - i wouldn't recommend anything higher than 20%. The bleaching agent should be some sort of peroxide.

Crest WhiteStrips: The crest whitestrips are made by a proven dental company with much research to support their product. Many products on the market are not as committed to research as is crest. These are safe and effective.

Yes: Yes, they are safe under dentist guidance.