Just want to know if there is any medical reason why a 6 year old girl's armpits can be smelly and whether she can use deodorant , and if yes what type is advisable ?

Basics: The odor producing glands of the armpits are under the hormonal influence of the adrenal glands that sit atop the kidneys. Their output is not directly tied to puberty & can sometime be active this early. Any deodorant she finds acceptable is okay. In girls, emergence of nipple enlargement on one or both sides is the 1st sign of puberty.