Child Benefit (CHB) can be paid for up to eight weeks if the wee one is absent from the claimant and no-one else has made a claim. It can continue to be paid after that period if the claimant continues to contribute at least the same as the rate of CHB being paid.
Your son should put in a claim as soon as possible as the claim can be backdated only three months.
Your son should give the date from which he has been looking after the two wee ones and if there is any evidence submitted to the Court to confirm that, include copies with the claim.
The Child Benefit Office (it was a Centre in my day) will contact the current claimant to get her to withdraw her claim and if she refuses or doesn't reply, they can decide who has the better entitlement to the CHB.
You find the claim form here,
https://www.gov.uk/government/publications/child-benefit-claim-form-ch2