Child benefits is a social security payment owned by the parents and not the child. On your Zakat anniversary, any money that you have with you should be included in your cash balance for Zakat purposes, irrespective of whether it came from salary, benefits, rental income etc.
Q: I have bank accounts for both my children and when I receive child benefit it goes to that account. am I liable to pay zakat on that saving they are not of balig age.
A: If the money goes into the ownership of the children, you do not have to pay Zakat on it.