The two trust funds (which is where the relevant parts of your estate will be held) are entirely separate. They should hopefully have been drawn up in such a way that money from those funds may be used, at the discretion of the trustees, for specified purposes prior to the beneficiaries attaining the age of 25. (For example, if your son or daughter was attending university at the time of your death you might want them to be able to receive some of the money straight away in order to continue with their studies, rather than have the money at 25 after having been forced to leave university, through lack of funds, a few years earlier).
However each child will receive whatever's left in their own trust fund (which will be completely separate to that of their sibling) when they attain the age of 25.