Any child that can understand and speak a language can be hypnotised. Ideally the child should be 7 or above. But I’ve had children brought to me who were just five years old and if they can understand spoken language and reply with limited vocabulary they can be hypnotised.
However, the induction methods used for child hypnosis are vastly different from classic inductions used for adults. That is because children tend to be distracted easily and induction methods for children take that fact into consideration.