Melanie - my reading of that clause would be that each child receives their share as they turn 25 years. However, I would never recommend to a client to include such a clause - I believe such a clause creates what is called a "discretionary trust". Such trusts can have significant tax implications and can be created unwittingly when parents seek to prevent inheritance until their children are older. Once inheritance occurs before 21 years their are usually no discretionary trust implications.
Now, I'm saying all this without knowing any information bar what you posted, so please ensure you go to a solicitor if you are thinking of making a will.