Puberty comes in two stages: the first is when the adrenal glands kick in and children get hairier and develop body odour. The second stage is when their gonads start to produce sex hormones and they go on to have periods, wet dreams etc.
The adrenarche can happen from birth, so potentially babies can have bo.
Mostly children from about 5 have adrenarche.
This does not mean they will have their menarche earlier, or have their testicles drop etc earlier.
The two are not linked, but usually the adrenals kick in about three years before the gonads start to produce any hormones.
Using a deodorant that actually works is VERY important for children, if they are stinky. It's very bad for their self esteem to be known as the smelly one. If the adrenals have started to kick in, then just washing and wearing clean clothes won't cut it- they will be smelly. You don't want to get into a situation where they aren't doing sports cos they have self consciousness about how they smell.
Alum is the salt in pit rock and can be bought in chemists and made into a solution - this works well for sweaty feet also.
But why not use a nice smelling deodorant, for mild smells, or anti perspiration if sweating is noticeable.
We use dr organcis from holland and Barrett, also toms of Maine, also welda, but I use Sure and Vaseline as I sweat more than my kids.
We all smell lovely!