I think your dentist is right NicNac, my DD 4.5 still sucks hers, have tried plasters making it taste bad, gloves but nothing lasts. I am just working on reducing the time, so only at bed time and in the car etc. The thumb doesn't stay in when she sleeps, it drops out. Not everyone who sucks their thumb has teeth issues (my sister didn't and dentist wasn't concerned at last check up) so I am taking a what will be will be approach.