If this is a case for a minor child and if a dedicated account is required, there are very strict guidelines on how that money is spent. It can only be spent on medical or educational needs, not on food, clothing or shelter. It also has to be kept separate from the other funds. You have to keep receipts for everything you spend. It also can be left to accumulate and is excluded from the $2000 resource limit. If you as payee don't follow the dedicated account rules, then you, as payee, owe the money back personally.

Wouldn't it be nice to have a chunk of money for post-secondary education for your disabled child when they turn 18?

If a dedicated account is NOT required, then the lump sum amounts are excluded from the $2000 resource limit for 9 months. You can spend that money on food, clothing, shelter, personal items. Any needs of the child.

There will be three kinds of reviews done for an SSI child: continuing disability which will absolutely be done at age 18 and could be done earlier; redeterminations of income and resources of the parents and child, probably annually; rep payee accountings done annually which include accounting of both the monthly benefits and the dedicated account funds. You also should be reporting parental income every month, especially wages. Giving a low wage estimate is a common reason for large overpayment of benefits. You don't want to be getting a bill next year for overpayment. Best if you open a separate bank account for the monthly benefits. Don't mingle it with the dedicated account rules. Follow the strict guidelines for dedicated account.

Retroactive benefits for children is not an opportunity to pay your own past due debts or to be used to provide basic necessities for your child.