There are many options you can choose from when investing for children or grandchildren. It is likely that you will want to choose the one that gives you the best combination of access, investment choice, and tax efficiency. The Guide to Investing for Children has a useful comparison that you can use to make up your mind on which option to choose.
Investing for children can be a difficult decision with factors such as whether you would want them to have access to the money at 18, whether you need the money to benefit their education before that, and if grandparents, then whether you put the investment in the name of the child, the parent or yourself.
Leaving a Trust to children has additional complications such as divorce, misuse of money by the child(ren), lack of expertise of the Trustees in managing the Trust etc. These are all important factors to consider before deciding precisely how to invest for children with your surplus assets.
That is of course assuming that you have taken the decision, and have complete understanding of the fact that you have enough assets for the rest of your life, to be able to give away money at this time.