"Kiddie" relaxers are currently allowed to be sold in the UK. Many relaxers contain sodium hydroxide, an extremely caustic chemical which breaks down the hair strand to permanently make it straighter.
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A "kiddie" relaxer is called child friendly, however, it is generally just a slightly milder relaxer.
Relaxers can permanently damage the hair, causing the hair to break. Many parents feel the need to use these on children as young as 3 years old so they will fit beauty standards, and although understandable, it can be dangerous and result in severe scalp burns and hair loss, and be incredibly difficult to reverse. Just breathing a small amount of sodium hydroxide can permanently damage your lungs. Children shouldn't have this forced on them.