Your ex remains an "eligible person" under the NSW Succession Act, so the first step is to make sure you have a property division with the ex, and include in any Binding Financial Agreement appropriate clauses to help prevent the ex from making a claim. There are other things that can be done, depending on the amount of assets involved and the chance of that happening.

Normally anything left to someone who is under 18 at your death can be used for their benefit, for things such as education, support etc, and it would be your executor who would decide what should be paid.You should appoint someone you trust, and give them a gift instead of "commission" or payment. One option is even to appoint your ex with or without another personif you die before your son reaches 18 or 21 (and only for the period until he reaches say 21 etc) , but if you die after that, to appoint your son himself. Your executor is bound by the terms of the will and the trusts set up by it.

You canput inspecific terms of the trust which operate as binding directions to your executor, but beware of "ruling from the grave" too much...you don't know when you will die, you don't know who will survive you,and you can't assume you will own then what you do now.

Any good lawyer/estate planner will get details of your assets, super,any other financial resources (eg the life insurance), exactly who owns each thing, details of your family, and then what you want to do, and only then consider the best way to achieve your objectives in a costs effective way.