There is no issue investing money for your grand-children, however, we do need to look at how this will affect your pension. Centrelink treats the gift as a “disposal” of an asset. There are limits on how much asset you can dispose of and still retain your current pension entitlement.
There are two gifting limits as follows: A person can dispose of assets of up to $10,000 each financial year,  and an additional disposal limit of $30,000 over a five-financial-years rolling period.
Where the above rules are not met, Centrelink will consider the excess gift as a deprived asset. This means the excess component will be counted as an asset and income will be assessed for 5 years from the time of the gift, even though you no longer have access or use of the asset. 
Leola, we need to determine the amount of money you wish to invest into this trust. If the amount you are considering is within the gifting limits then you can proceed and there will be no impact on your Centrelink benefits, you will, however, need to notify Centrelink that you have made a gift.
If the gift exceeds the limits then it could be more beneficial for you to retain these excess funds in an investment account and make a specific bequest in your Will in favour of your grand-children. Alternatively, you could consider using Insurance Bonds which can nominate your grand-children as beneficiaries.
Leola, it is important that you seek the advice of a qualified Financial Planner before making any final decision so that your particular financial circumstances can be assessed..