Thanks for your enquiry.
Provided the Executors named in your Will know where to find your grandchildren, there is no scary issues you need to be made aware of. As you live in the UK, your Estate and in particular the Inheritance Tax laws of the UK will apply.
However, I am not sure if you have already stipulated this, but the youngest age for which a person can inherit under a Will is 18. Alternatively, if you wish, you can stipulate that your grandchildren do not inherit their legacy until they reach 21. If anything happens to you before the youngest reaches 18/21, it will be up to your Executors (should be 2 family members) to become Trustees, and whom will be responsible for investing the monies until they reach 18/21.
I hope this assists and sets out the legal position.