If your grandchildren are already living with you on an informal basis, then it is sensible to put this on a more formal footing and ensure that you have parental responsibility and therefore the ability to make decisions about routine medical care, schooling or even if you can take them on holiday as well as more major or urgent decisions.
If your grandchildren have already been living with you for three years or more, then you do not need to ask the court’s permission to make an application.