Thank you for your query.
This is contact dermatitis
Apply a moderately potent topical steroid
cream twice daily for seven days.Use a good aloe vera
cream along with calamine
lotion to that area twice daily.You have to stop thumb sucking or else the lesion will recur.
If you have any further queries you are free to consult me.
Hope this answers your query.
Wishing your child a speedy recovery.
Have a nice day.