I agree with M0nica. I have made 4 wills in my life: 2 were very straightforward and made on will forms (when I had no children, and again when they were adults). 2 were more complicated: appointing testamentary guardians and making other provision when the children were under 18; and the other as you wish, leaving money split between children and grandchildren.
For the latter, you do need a solicitor, unless you are happy that your children will honour your wishes.
A note of caution on that: should any of your children be in need of long-term care or benefits at the time they inherit, they cannot simply give away the money they inherit. Their benefits will be calculated on the money they receive from the will, and no notice taken of wishes expressed outside of that (I should say that which benefits are affected is also quite complicated, not all are, just saying it may not be simple!)