What can i use for my 8 year old smelly armpits?

Whatever you wish: The apocrine (odor) glands of the armpit are under the control of the adrenal glands. They may become active months to years before the start of true puberty. Any deodorant the kid is comfortable with can be used. You can test any product before use by applying it to the pad of a Band-aid and placing it on the skin overnight. If no redness or other reaction it should be fine.