I think the position is that although you should always try to obtain proof of all income, including child benefit, you can apply para 12.2.15 of the financial eligibility regs, which reads:
15. As long as evidence is obtained of the main source of income, it will be unnecessary to carry on and obtain evidence of small additional amounts of income that are unlikely to affect eligibility. Where the client is in prison, it will be unnecessary to obtain documentary evidence of their prison income (although the amount stated by the client should be added in to the assessment) as the amounts that can be earned are strictly limited.
As your file will show how many dependant children there are in the household and child benefit at a fixed rate is almost always in payment for each child, you can use this rule to get round having the proof, as long as the CB is included in the calculation and the client is still eligible.. Always better to get proof if possible though