Babies and toddlers suck their thumbs for comfort. It also comforts them and is actually a calming technique. While it is not uncommon to see young children walking with their thumb in their mouth, especially when they are upset or angry, thumb sucking can cause and even cause dental and language problems in older children.
Most children stop thumb sucking between the ages of 2 and 3, but children who still suck their thumbs after the age of 5 may be normal but should be taken to the dentist. You can get information about finger sucking prevention via https://www.amazon.co.uk/Age-2-7-Stop-Thumb-Sucking/dp/B00VQU2DZS.
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An effective way to limit thumb sucking
Identify the trigger: First you need to know and understand when and why the child sucks his thumb? When is he upset or angry? When did you press him to study? Once you understand the trigger, you can plan ahead to deal with the situation between the two of you and decide that he won't suck his thumb.
Involve the child: Discuss patiently with the child and assess their willingness. Break the habit together and gently remind him every time you see him doing it that he broke his promise.
Distraction: The child will be angry, insecure, and angry and this situation cannot be avoided. However, children can easily get distracted, so you can get their attention by offering something else – like a stuffed animal or about a car or cartoon – that will immediately draw their attention to the object at hand.