Hi Didry,
That is a great question. It is often difficult for a child under the age of three to tap on themselves. With children under the age of three, I generally tap on them myself, or have their parent do the tapping for them. This means tapping gently on the EFT tapping points while they are upset, or while they are telling their "story." For toddlers (or even infants) who can not always verbalize what is wrong, you can tap on them (as described above) while saying things that you think may be causing any distress. (Even though I'm tired/hungry/scared/fussy/overstimulated/my tummy hurts/got too hot/too cold/afraid/Mommy/Daddy/sibling is upset/etc...)
There is no required age for EFT to work or be effective. I urge you to try it on children of any age. There is no harm in using EFT on young children.
By age five or six, many children can even start tapping on their own. I have heard stories from parents of my little clients, that the children will tell them they want to tap, or they will take the initiative on their own to tap "all by themselves," which I think is wonderful. They know it works.
My own personal opinion is that you want children to learn to feel their feelings. So EFT should not be used to numb everything. It is safe and effective in reducing or eliminating emotional stress.
Best wishes!